This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This rticle ppered in journl published by Elsevier. The ttched copy is furnished to the uthor for internl non-commercil reserch nd eduction use, including for instruction t the uthors institution nd shring with collegues. Other uses, including reproduction nd distribution, or selling or licensing copies, or posting to personl, institutionl or third prty websites re prohibited. In most cses uthors re permitted to post their version of the rticle (e.g. in Word or Tex form) to their personl website or institutionl repository. Authors requiring further informtion regrding Elsevier s rchiving nd mnuscript policies re encourged to visit:

2 Author's personl copy Reserch in Developmentl Disbilities 32 (2011) Contents lists vilble t ScienceDirect Reserch in Developmentl Disbilities How much chnge is true chnge? The smllest detectble difference of the Preschool Imittion nd Prxis Scle (PIPS) in preschoolers with intellectul disbilities of heterogeneous etiology M. Vnvuchelen,b,c, *, C. Vochten d Deprtment Helth Cre, PHL University College, Belgium b Deprtment of Rehbilittion Sciences, V.U.B., Belgium c Deprtment of Rehbilittion Sciences, K.U.Leuven, Belgium d Student Counseling nd Guidnce Center, Hsselt, Belgium ARTICLE INFO ABSTRACT Article history: Received 11 September 2010 Accepted 20 September 2010 Keywords: Imittion Preschool child Intellectul disbilities Relibility Smllest detectble difference Preschool Imittion nd Prxis Scle (PIPS) The teching of imittion skills is often the first step in interventions for young lerners with intellectul disbilities. The min gol of this study ws to determine the smllest detectble difference (SDD) t 95% confidence of the Preschool Imittion nd Prxis Scle (PIPS) in preschoolers with intellectul disbilities. Two rters independently scored videotpes of the imittion performnce of 44 preschoolers (27 with Down syndrome, 10 with Non-Specific Mentl Retrdtion nd 7 with Low-functioning Autism) between 13 nd 58 months of ge (men ge 39.6 months, SD 11.9 months). Results reveled tht the PIPS demonstrted cceptble interrter relibility on item level (weighted kpp vlues rnged from 0.52 to 0.96) nd scle level (ICC = 0.986; 95% CI: ). The SDD of the PIPS ws 7.2%, indicting tht the chnge score rted by different rters for n individul child with n intellectul disbility is vlid nd tht the PIPS cn be used by erly interventionists nd reserchers s n outcome mesure to determine children s mturtion or improvement. ß 2010 Elsevier Ltd. All rights reserved. 1. Introduction Motor imittion (herefter imittion ) is powerful mechnism to lern motor skills nd to fcilitte comprehension of other persons ctions nd mentl sttes (Hurley & Chter, 2005). The term imittion hs mny definitions (Sevlever & Gillis, 2010). There re however two connottions in which it is most commonly used. The first defines imittion s the cpcity of n individul to replicte n observed motor ct (Prinz, 2002). The second defines imittion s the cpcity to cquire, by observtion, new motor behviour nd to repet it using the sme movements employed by the demonstrtor (Tomsello, Crpenter, Cll, Behne, & Moll, 2005). Imittion skills occur without direct trining in typiclly developing children nd ply n importnt prt in the development of ge-pproprite prxis nd socil skills (Msur, 2006). Imittion voids time-consuming tril-nd-error lerning. In reproducing the exct nd detiled fetures of the demonstrtor s ctions, children re likely to successfully complete the intended ctions, even with limited understnding of their purpose. Fithful copying cn be used to disentngle the gol of n ction to be imitted when it is not completely cler to the child (Lyons, Young, & Keil, 2007). * Corresponding uthor t: Sterrebos 111, B 3512 Stevoort, Belgium. Tel.: ; mobile: E-mil ddresses: MVnvuchelen@mil.phl.be, Mrleen.Vnvuchelen@fber.kuleuven.be (M. Vnvuchelen) /$ see front mtter ß 2010 Elsevier Ltd. All rights reserved. doi: /j.ridd

3 Author's personl copy M. Vnvuchelen, C. Vochten / Reserch in Developmentl Disbilities 32 (2011) Consequently, the precision of the perception-ction coupling, expressed in the ccurcy of imittion performnces is n importnt criterion for children s ptitude to lern new skills. The cquisition of skilled cts, lerned effortlessly by more ble young children, is crucil spect of the overll development of children with developmentl disbilities. The imittion problems mong lerners with intellectul disbilities (Mcedoni-Luksic et l., 2009; Vnvuchelen, Feys, & De Weerdt, in press) nd with utism spectrum disorders (Perr et l., 2008; Vnvuchelen, Roeyers, & De Weerdt, in press-; Vnvuchelen, Roeyers & De Weerdt, 2011; Willims, Whiten, & Singh, 2004; Zchor, Ilnit, & Itzchk, 2010) my prevent them from lerning the skills tht typiclly developing children lern in nturl environments. Adults cnnot directly tech children ll the skills needed for dult life. Approprite lerning depends primrily on the child himself or herself tking on the responsibility for lerning throughout the dy by imitting others, engging others, nd exploring the potentil of the vrious environments. Therefore, the teching of imittion skills is often the first step in interventions for young lerners with intellectul disbilities (Ber, Peterson, & Shermn, 1967; Brown, Brown, & Poulson, 2008; Brown, Pece, & Prsons, 2009; Meht, Pnde, & Bhrgv, 1991) nd utism spectrum disorders (Lovs, 1987; Sthmer, Ingersoll, & Crter, 2003; Rogers, 1998). Behviourl intervention pproches to tech imittion skills with the use of video modeling (Tereshko, McDonld, & Ahern, 2010) nd live modeling, e.g., Discrete Tril Trining (Brown et l., 2008) nd Reciprocl Imittion Trining (Crdon & Wilcox, in press; Ingersoll, 2010; Ingersoll & Gergns, 2007; Ingersoll & Llonde, 2010; Ingersoll, Lewis, & Kromn, 2007; Ingersoll & Schreibmn, 2006) re essentil methods to improve the lerning potentil of intellectul disbled children with imittion problems. Although the existing literture on effects of imittion teching provides evidence tht this kind of erly intervention strtegy improves the prognosis of the children, reserch findings re bsed on methodologiclly wek studies with few prticipnts nd the bsence of control group, n lterntive intervention strtegy nd systemtic imittion ssessment. The Preschool Imittion nd Prxis Scle (PIPS) is designed to ssess the ccurcy of bodily (ctions without objects) nd procedurl (ctions with objects) imittion performnce t preschool ge (Vnvuchelen, Roeyers, & De Weerdt, 2011b). Imittion ge-equivlent scores re derived from PIPS scores of 654 typiclly developing children between 12 nd 59 months of ge (Vnvuchelen, 2009). Evidence of the vlidity nd relibility of the PIPS in helthy preschoolers hs been provided in severl studies (Vnvuchelen et l., 2011b; Vnvuchelen, Roeyers, & De Weerdt, in press-b). However, relibility tht is estblished on individuls from one popultion, e.g., typiclly developing preschoolers, cnnot utomticlly be ttributed to nother popultion, e.g., preschoolers with intellectul disbilities. This hs been termed popultion-specific relibility. The fctor of intellectul disbility cn lter the wy child responds to mesurement nd the consistency with which exminers cn tke the mesurement (Portney & Wtkins, 2009). Furthermore, to be cliniclly meningful n imittion ssessment must be relible enough to evlute outcomes of therpeutic intervention, such s teching imittion skills. Knowing tht the PIPS hs chieved certin level of psychometric dequcy to identify preschoolers with imittion problems (Vnvuchelen et l., in press-) sys nothing bout its sensitivity to tretment-relted chnges in child functioning. The intent of the ssessment, e.g., mking dignosis versus testing the efficcy of n intervention, cn potentilly result in different estimte of importnt chnge (Normn, Slon, & Wyrwich, 2003). To determine if the imittion ptitude hs chnged, n exminer must know wht prt of the difference between children s mesurements is ttributble to rel chnge, nd wht prt is due to mesurement error. Relibility cn be reported in reltive or bsolute terms. Reltive interrter relibility sttistics indicte the degree of ssocition between two or more rtings (e.g., intrclss correltion coefficients or ICCs), but they do not provide clinicl guidnce for ssessing rel chnges t n individul child level. If the PIPS is used in intervention studies of preschoolers with intellectul disbilities, the knowledge of the stndrd error of mesurement (SEM) nd the smllest detectble difference (SDD) is importnt to scertin which children show true imittion chnge prt from differences due to mesurement errors (Atkinson & Nevill, 1998; Portney & Wtkins, 2009). The SEM nd SDD provide indictions of bsolute relibility. The precision of the mesurement is expressed s the SEM. The SDD is the mount of chnge in vrible tht must be chieved to reflect true difference (Portney & Wtkins, 2009). This is the smllest mount of difference tht psses the threshold of error for specific instrument, e.g., the PIPS, nd ppliction, e.g., in preschoolers with intellectul disbilities. The present rticle provides evidence of reltive nd bsolute interrter relibility of the PIPS in preschoolers with intellectul disbilities of heterogeneous etiology. 2. Methods 2.1. Prticipnts Forty-four preschoolers (19 femle nd 25 mle) between 13 nd 58 months of ge (men chronologicl ge 39.6 m, SD 11.9 m) with n intellectul disbility (27 with Down syndrome, 10 with Non-Specific Mentl Retrdtion nd 7 with Lowfunctioning Autism) prticipted in this study. Nonverbl mentl ge ws determined with the Dutch modifiction of the Byley Scles of Infnt Development (BSID-II-NL; Vn der Meulen, Ruiter, Lutje Spelberg, & Smrkovský, 2000) nd rnged between 8 nd 43 months of ge (men nonverbl mentl ge 25.6 m, SD 9.7 m). The children with Down syndrome nd with Non-Specific Mentl Retrdtion were recruited from home-bsed child development progrms for children with intellectul disbilities, respectively specil schools for children with lerning disbilities. The children with Low-functioning Autism were dignosed ccording to multidisciplinry clinicl consensus

4 Author's personl copy 182 M. Vnvuchelen, C. Vochten / Reserch in Developmentl Disbilities 32 (2011) clssifiction (University Autism Clinics) in ddition to positive ADOS-G-clssifiction (Lord, Rutter, DiLvore, & Risi, 2003). All fmilies gve written informed consent for the prticiption of their child Preschool Imittion nd Prxis Scle (PIPS) The theoreticl frmework nd deductive test construction pproch of the Preschool Imittion nd Prxis Scle (PIPS) hve been described in detil elsewhere (Vnvuchelen et l., 2011b). To recp briefly, different ctiontypes considered to be importnt s unrvelled in reserch in prxic dults were selected to tp brod rnge of possible imittion mechnisms: ction types with different effects (slient environmentl nd internl), representtionl levels (meningful nd non-meningful; gol directed nd non-gol directed), temporl complexities (single nd sequentil) nd visul monitoring possibilities (trnsprent nd opque). The PIPS consists of 10 tsk ctegories with ech three tsks: six gesturl, three procedurl nd one fcil. Imittion tsks which re possible to be performed by young children but unlikely to be exhibited spontneously were selected (Vnvuchelen et l., 2011b). The 10 tsk ctegories nd 30 PIPS tsks re described in Tble 1. Imittion performnces on ech tsk re scored on 3 5 point scle in ccordnce with the criteri of the scoring system of the PIPS, which evlutes the sptiotemporl resemblnce between the modelled nd copied ction (Vnvuchelen, 2009). To illustrte this system, we exemplify the scoring of the tsk to pretend to comb your hir with n imginry comb. Score 4 is given if the child hs used symbolic grip nd hs performed repetitive ction on both sides of the hed. Score 3 is given if the child hs used symbolic grip nd hs performed repetitive ction on one side of the hed or single ction. Score 2 is given if the child hs used body-prt-s-n-object grip nd hs performed repetitive ction on both sides of the hed. Score 1 is given if the child hs used body-prt-s-n-object grip nd hs performed repetitive ction on one side of the hed or single ction. Score 0 is given is the child hs performed nother ction or hs refused to imitte. The finl PIPS score (rnging between zero nd 81) is reflection of the ccurcy of the child s imittion performnce (Vnvuchelen, 2009). Interrter relibility of the PIPS items (weighted kpp vlues rnged from 0.64 to 1) nd the totl score (intrclss correltion coefficient ICC = 0.995; 95% CI: ) hs been estblished in popultion of typiclly developing children. In this popultion, the SDD ws 5.6% of the possible score rnge of the PIPS (Vnvuchelen et l., in press-b). For the test dministrtion, ech child ws individully ssessed in quiet room nd ws seted t tble in front of the trined exminer. The dministrtion of the 30 tsks of the PIPS ws in ccordnce with the guidelines for item instruction of the PIPS (Vnvuchelen, 2009). Before demonstrting ech tsk, child s ttention ws ttrcted by ddressing the child by nme. Only the verbl instruction (Nme), you do this too ws given Dt nlysis All nlyses hve been performed using the sttisticl softwre SPSS (version 16.0). P-vlues smller thn 0.05 were considered s significnt Descriptive sttistics Simple descriptive sttistics (men nd stndrd devition) re used to describe the bsic fetures of the PIPS in this study Reltive interrter relibility To determine reltive interrter relibility, the imittion performnce of the children ws videotped. Two trined exminers scored the performnces on the videotpes independently. Reltive interrter relibility on individul item level were exmined using Cohen s weighted kpp vlues. The interprettion of kpp vlues ws done ccording to Fleiss (Fleiss, 1981): below 0.40 greement by coincidence; between 0.40 nd 0.60 moderte; between 0.61 nd 0.75 good nd bove 0.75 excellent greement. Percentges of greement were lso determined for ll items: 70% or higher ws considered s cceptble. Reltive interrter relibility on scle level ws exmined with the intrclss correltion coefficient (ICC). The option treting subjects s well s rters s rndom effect ws pplied. This resulted in two-wy rndom effects model with mesures of bsolute greement. The interprettion of the ICC ws done ccording to Portney nd Wtkins (2009): vlue below 0.75 is indictive of poor to moderte relibility nd vlue bove 0.75 of good relibility. Blnd nd Altmn s limits of greement on the scle score (men of the differences between rtings 2SD) were used to ssess the strength of greement between the rters (Blnd & Altmn, 1986) Absolute interrter relibility Absolute interrter relibility ws exmined by the smllest detectble difference (SDD) to determine whether the chnge scores rted by different rters of n individul child were vlid (beyond rndom errors) t the 95% confidence level.

5 Author's personl copy M. Vnvuchelen, C. Vochten / Reserch in Developmentl Disbilities 32 (2011) Tble 1 Description of the 10 tsk ctegories nd 30 items of the Preschool Imittion nd Prxis Scle (PIPS) presented in stndrdised order. Item nr T Ctegory Dimension Tsk description Score 1 P so-p1 GDP Rise toy ber by pulling cord P so-p2 GDP Put wooden block on top of your hed P so-p3 GDP Switch on lmp in toy niml with your forehed G i-mg1 SIB Perform the gesture to wve good-bye G i-mg2 SIB Perform the gesture to show something with n outstretched 0 2 hnd in supintion 6 G i-mg3 SIB Perform the gesture to beckon with the index finger G si-nmg1 SIB Rise your outstretched rm till 908 nteflexion nd mke circle with the index finger nd thumb 8 G si-nmg2 SIB Rise your outstretched rm till 908 nteflexion nd stretch out your fingers 9 G si-nmg3 SIB Rise your outstretched rm till 908 nteflexion, hold up the little finger while ll the other fingers nd the thumb re bent 10 P sq-p1 NGDP Open the box, put the lid on the tble, turn the box upside-down, put the block on the bottom of the box 11 P sq-p2 NGDP Tke the block from the bottom of the box, turn the box in norml position gin, close the box, put the block on the lid of the box 12 P sq-p3 NGDP Tke the block from the lid of the box, open the box, put disc into the box, close the box, put the block gin on the lid of the box 13 G t-mg1 SIB Pretend to comb your hir with n imginry comb G t-mg2 SIB Pretend to open n imginry door with n imginry key G t-mg3 SIB Pretend to brush your teeth with n imginry toothbrush G bi-nmg1 SIB Plce one fist on top of the other 17 G bi-nmg2 SIB Extend the index fingers of both hnds while the other fingers nd thumbs re bent, nd bring the top of the index fingers towrds ech other 18 G bi-nmg3 SIB Open one hnd in verticl position nd touch the top of the fingers with the plm of the other hnd in horizontl position 19 G f-nmg1 SIB Extend your index finger nd touch the top of your nose 20 G f-nmg2 SIB Touch your lower lips with the nils of your thumbs 21 G f-nmg3 SIB Extend the index finger of your left hnd nd touch your right cheek nd extend the index finger of your right hnd nd touch your left cheek 22 P so-p1 GDP Turn cup upside-down nd ply drums on it with two spoons P so-p2 GDP Remove the cp of doll nd put shoe on the hed of the doll P so-p3 GDP Put toy cr in bed, turn it upside-down nd tuck it in with blnket G sq-nmg1 SQB Hit the tble with the plm of your hnds, cross the rms nd hit the tble gin, return to the originl position nd hit the tble once more 26 G sq-nmg2 SQB Hit the tble with one hnd in supintion, turn the hnd in prontion nd hit the tble gin, clp in the hnds, hit the tble with the plm of both hnds 27 G sq-nmg3 SQB Hit the tble with both hnds in supintion, turn the hnds in prontion, hit the tble gin, clp in the hnds, hit the tble with the plm of both hnds once more 28 F f1 SIB Shke the hed, eyes closed to sy no, with n expression of dispprovl F f2 SIB Look ngry with frown of the eyebrows F f3 SIB Nod quickly with your hed nd show n expression of hppiness 0 2 T, type of ction (P, procedurl imittion; G, gesturl imittion; F, fcil imittion); so-p, substituted-ctions-upon-objects; i-mg, intrnsitive meningful gestures; si-nmg, single non-meningful hnd postures; sq-p, ction-sequences-upon-objects; t-mg, trnsitive meningful gestures; bi-nmg, bimnul non-meningful hnd postures; f-nmg, non-meningful hnd postures to the fce or hed; so-p, ctions-upon-substituted-objects; sq-nmg, sequences of non-meningful hnd postures; f, fcil expressions; GDP; gol directed procedurl imittion; NGDP, non-gol directed procedurl imittion; SIB, single bodily imittion; SQB, sequentil bodily imittion. First, the stndrd error of mesurement (SEM) for the two seprte rtings (SEM first nd SEM second ) ws clculted on the bsis of the stndrd devition (SD) nd the ICC: SEM = SD H1 ICC, which ws used to quntify the mount of rndom mesurement errors. Assuming tht mesurement errors re distributed normlly, the corresponding 95% confidence intervl, in which the true score is expected, ws 1.96 SEM (Portney & Wtkins, 2009). There re no cler criteri for n cceptble SEM-vlue vilble. So, how much rtings cn be devint from one nother will be question of judgment. Preferbly, this should be defined in dvnce (Blnd & Altmn, 1986). We expressed the SEM s percentge of the possible score rnge of the PIPS (from 0 to 81). We postulted tht vlue lower thn 10 percent of the possible score rnge of the PIPS, which mens SEM score below 8.1, nd difference between the percentges of two SEMs smller thn 1 percent would be stisfctory (Vn Blen, Odding, vn Woensel, & Roebroeck, 2006). Then, the smllest detectble difference (SDD) ws clculted using the following formul: SDD = 1.96 H(SEM first 2 +- SEM second 2 ). If the difference between both SEMs ws smller thn 1 percent, we ssumed tht they were equl nd simplified the formul to: SDD = 1.96 SEM H2 (Portney & Wtkins, 2009). When interpreting the SDD the following

6 Author's personl copy 184 M. Vnvuchelen, C. Vochten / Reserch in Developmentl Disbilities 32 (2011) criteri were used. First, when the SDD vlue ws lower thn 10% of the possible highest score of the PIPS, which mens SDD score below 8.1, the PIPS ws considered cceptble (Kropmns, Dijkstr, Stegeng, Stewrt, & de Bont, 1999). Second, the SDD should be close to 0.5 stndrd devition for discriminting the threshold of chnge (Normn et l., 2003). 3. Results 3.1. Descriptive sttistics The men totl PIPS score ws 28.7 (SD 17.9) ccording to the first rter nd 29.2 (SD 19.2) ccording to the second rter Reltive interrter relibility Tble 2 provides the weighted kpp vlues nd percentges of greement for the interrter greement of the 30 individul item scores. Among the 30 PIPS items, 18 reveled n excellent interrter greement for individul item scores, 4 hd good greement, nd 3 hd moderte greement. The kpp sttistics of 5 items could not be computed, becuse of the skewed distribution of the dt. However, the percentge of greement of these 5 items ws obtined: 70% (t-mg1), 72% (t- MG2), 70% (t-mg3), 84% (f-nmg2) nd 79% (sq-nmg1). The interrter relibility of the PIPS scle scores ws high (ICC = 0.986; 95% CI: ). Fig. 1 plots limits of greement clculted from interrter dt of the PIPS scle scores. Plotting PIPS mens of the two observtions ginst the interrter difference for ech prticipnt does not give ny indiction tht mesurement errors vry systemticlly over the rnge of possible scores Absolute interrter relibility The SEM of the first rter ws 2.1 (2.6% of the possible score rnge of the PIPS) nd of the second rter 2.3 (2.9% of the possible score rnge of the PIPS). According to the dt of the first rter, there is 95% chnce tht the true men PIPS score lies between 24.5 nd 32.2; nd ccording to the second rter between 24.6 nd The difference between both ws 0.2 (0.3% of the possible score rnge of the PIPS). Since the difference between the percentges of two SEMs ws smller thn 1%, we hve used the SEM of the first rter to determine the SDD. The SDD ws 5.8 (7.2% of the possible score rnge of the PIPS). Tble 2 Weighted kpp vlues nd percentges of greement for interrter greement of 30 individul item scores of the Preschool Imittion nd Prxis Scle (PIPS). Item number Tsk Weighted kpp vlue Asymptotic stndrd error Percentges of greement 1 so-p % 2 so-p % 3 so-p % 4 i-mg % 5 i-mg % 6 i-mg % 7 si-nmg % 8 si-nmg % 9 si-nmg % 10 sq-p % 11 sq-p % 12 sq-p % 13 t-mg1 70.4% 14 t-mg2 72.7% 15 t-mg3 70.4% 16 bi-nmg % 17 bi-nmg % 18 bi-nmg % 19 f-nmg % 20 f-nmg2 84.1% 21 f-nmg % 22 so-p % 23 so-p % 24 so-p % 25 sq-nmg1 79.5% 26 sq-nmg % 27 sq-nmg % 28 f % 29 f % 30 f % Kpp sttistics could not be computed, becuse of the skewed distribution of the dt.

7 Author's personl copy [()TD$FIG] M. Vnvuchelen, C. Vochten / Reserch in Developmentl Disbilities 32 (2011) Fig. 1. Blnd nd Altmn s limits of greement for the Preschool Imittion nd Prxis Scle (PIPS) scle score on the interrter dt. 4. Discussion Imittion is one of the pivotl skills tht young children use to lern from others. Trgeting the development of imittive skills in lerners with imittion problems my be n importnt focus in erly intervention. Interventions such s teching imittion skills tht result in child initition, explortion, nd ongoing enggement of the socil s well s the physicl world re likely to led to greter child lerning long term. These self-lerning behviours need to be identified s tretment gols nd exmined when ssessing children s response to tretment (Rogers, 1998). The Preschool Imittion nd Prxis Scle (PIPS) is relible nd vlid mesure to identify preschoolers with imittion problems (Vnvuchelen, Feys et l., in press; Vnvuchelen et l., in press-). To be cliniclly meningful the PIPS must be relible enough to evlute outcomes of n intervention, such s teching imittion skills. In clinicl settings, it is essentil to know by how much the score of one rter is likely to differ from the score of nother rter. If this is not enough to cuse problems in clinicl interprettion we cn use the two interchngebly. The present pper provides evidence for popultionspecific relibility, in prticulrly in preschoolers with intellectul disbilities of heterogeneous etiology. Rndom errors reduce the objectivity of the dt nd cn rise from inconsistencies in the dministering nd scoring protocol. Therefore, the exminers were trined in these protocols to prevent inconsistencies of scores. The interrter relibility ws moderte to excellent t the individul item level nd good for the PIPS totl score. However, the interrter relibility bsed on correltion coefficients, such s n intrclss correltion coefficient, provide n indiction of reltive relibility (Atkinson & Nevill, 1998). A high intrclss correltion does not lwys men tht the two rters gree. Correltions mesure the strength of reltion between two scores, not the greement between them. When dt of two rtings re plotted, there is perfect correltion if the points lie long ny stright line. But there is perfect greement only if the points lie long the line of equlity (Blnd & Altmn, 1986). Since correltions re highly influenced by the rnge of vlues in the smple, it is essentil to clculte the bsolute relibility, including the stndrd error of mesurement (SEM) nd the smllest detectble difference (SDD). The precision of the mesurement is expressed s the SEM. As postulted the SEMs of both rters were lower thn 10 percent of the possible score rnge of the PIPS, in prticulr 2.6%, respectively 2.9%. If the PIPS is used in intervention nd longitudinl studies, the knowledge of the SDD is relly importnt for erly interventionists nd reserchers. The SDD vlue is importnt to determine whether the chnge scores rted by different rters for n individul child fter trining or mturtion indicte rel chnge (i.e., beyond mesurement error) t the 95% confidence level. A sttisticlly significnt chnge between two rtings must be lrger thn the SDD (Portney & Wtkins, 2009). The SDD for the PIPS in preschoolers with intellectul disbilities ws 5.8 points or 7.2% of the possible score rnge of the PIPS, indicting tht two ssessments of the sme child should differ by more thn 6 points to reflect fctul imittion chnge. This smll SDD vlue met Normn et l. s (2003) criterion of n cceptble SDD. The SDD of 5.8 points ws clerly below 0.5 stndrd devition. This finding is consistent with the results of the SDD for the PIPS score in typiclly developing preschoolers (Vnvuchelen et l., in press-b). Some implictions of this study to clinicl prctise cn be mde. The SDD cn be considered s conservtive estimte of ptient s progress, identifying the smllest mount of chnge tht could be interpreted s ny improvement or decline. Therefore, using the SDD s criterion for improvement my be thought of s hving high specificity (voiding flse positives) but low sensitivity (finding mny flse negtives) (Portney & Wtkins, 2009). A criticl remrk hs to be mde. The present smple ws comprised of Dutch speking preschoolers with intellectul disbilities. However, two limittions should be noted. Directions for further reserch include: (1) the repliction of the

8 Author's personl copy 186 M. Vnvuchelen, C. Vochten / Reserch in Developmentl Disbilities 32 (2011) findings in other smples of preschoolers with intellectul disbilities, including those from other cultures nd (2) the investigtion of the utility of the PIPS in rndomised controlled tril to evlute the effectiveness of different intervention progrms for improving the imittion skills of young lerners with imittion problems. Acknowledgments This study ws prtilly supported by Foundtion Mrguerite-Mrie Delcroix grnt to M. Vnvuchelen. We would lso like to thnk ll children nd the stff of the University Clinics of Antwerp nd Ghent (Belgium) who prticipted in this reserch. We thnk the stff of the home-bsed child development progrms for children with mentl disbilities nd the stff of the specil schools for children with lerning disbilities, s well s the students Jolien Driessens (Deprtment of Rehbilittion Sciences K.U.Leuven, Belgium) nd Ellen Reynders (Deprtment of Helth Cre, PHL University College, Belgium) for their ssistnce with dt cquisition. References Atkinson, G., & Nevill, A. M. (1998). Sttisticl methods for ssessing mesurement error (relibility) in vribles relevnt to sports medicine. Sports Medicine, 26(4), Ber, D. M., Peterson, R. F., & Shermn, J. A. (1967). The development of imittion by reinforcing behviorl similrity to model. Journl Experimentl Anlysis of Behvior, 10, Blnd, J. M., & Altmn, D. G. (1986). Sttisticl methods for ssessing greement between two methods of clinicl mesurement. Lncet, 8(1), 8476, Brown, A. K., Brown, J. L., & Poulson, C. L. (2008). 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