Other Uses for Cluster Sampling

Transcription

1 Other Uses for Cluster Smpling Mesure hnges in the level of n ttriute Hypothesis testing versus intervl estimtion Type I n 2 errors Power of the test Mesuring ttriute t sme time in ifferent sites Exmple: Use of moern fmily plnning methos Mesuring ttriute t ifferent times in sme sites ssess the ourrene of unommon events Not reommene Exmple: neontl tetnus mortlity Other pplitions Mesure knowlege, ttitues n prties (KP) Deriving ontrols for popultion-se se-ontrol stuy 518 Use of Moern Fmily Plnning Methos Survey finings Surveys in Two Communities B B Shoul not hve te B No tion Type 1 error Truth B Type 2 error tion (or p-vlue) - set t.5 or less Shoul hve te usully set t 2% or less 1 - power of test - usully set t 8% or more 521 Use of Moern Fmily Plnning Methos Surveys in Two Communities Gol is to provie equl ess to moern FP methos in oth ommunities Comments of Lemeshow n Roinson* Power of the Test Community The smple size of 21 use y EPI my e too smll to voi mking Type 2 errors when ompring the proportion who re immunize t two ifferent time perios. True ifferene ws not etete If no ifferene -- No tion If ifferene -- tion to ress ifferene * REFERENCE 9 Lemeshow S, n Roinson D: Surveys to mesure progrm overge n impt: review of the methoology use y the Expne Progrmme on Immuniztion. Worl Helth Sttistis Qurterly 38, 65-75, Use of Moern Fmily Plnning Methos Surveys in Two Communities Use of Moern Fmily Plnning Methos Plnne Surveys in Two Communities Truth B B Community Survey finings B B No tion Shoul not hve te Shoul hve te tion 55% 4% Best estimte of perentge finings efore survey is one

2 Smple Size for Differene etween Two Proportions CSurvey - Input Vlues Estimte vlue of first proportion.55 Estimte vlue of seon proportion.4 Smple Size for Differene etween Two Proportions CSurvey - Output Vlues (2) CONFIDENCE INTERVLS OF PROPORTION DIFFERENCE 9% 99% Upper CL Desire level of onfiene Point Estimte Numer of lusters 3 Lower CL verge numer of persons per luster Rte of homogeneity 1 Low onverte to vlue tht orrespons to eff 2 p 2 -p 1 +/- [ x se(p 2 -p 1 ) ] p 2 -p 1 +/- [ 2.45 x se(p 2 -p 1 ) ] p 2 -p 1 +/- [ x se(p 2 -p 1 ) ] - inlue these vlue in grph to shre with eision-mkers or genies funing the survey prior to onuting the fiel stuy Vrine of Differene etween Two Proportions Definition Formul The vriility of the ifferene etween two smple mens, stte s proportions or perentges Estimte Confiene Intervl for Differene etween Prevlene Estimtes (%) Differene (%) v(p 2 -p 1 ) p 1 q 1 + p 2 q 2 n m x eff Numer of lusters verge persons per luster % 99% Confiene Level 528 Smple Size for Differene etween Two Proportions Smple Size n Deision-mking Stnr error of ifferene etween CSurvey - Output Vlues (1) Smple size for propose luster survey Upper onfiene limit Point estimte Lower onfiene limit Is smple size equte for stte nee?.57 se(p 2 -p 1 ) 3 Design effet for luster survey 2. set y progrm p 1 q 1 + p 2 q 2 n m n m p 2 -p 1 + [ t x se(p 2 -p 1 ) ] p 2 -p 1 x eff p 2 -p 1 - [ t x se(p 2 -p 1 ) ] p 2 -p 1 - [ t x se(p 2 -p 1 ) ] > True ifferene is 15% Differene (%) 9% 99% Confiene Level Community 55% 4% Is the survey instrument useful enough, even with wie onfiene intervls, to guie neessry eision-mking? 4 3 Exess in 2 Community in Current FP users (%) 1-1 tion Level tion in to estlish or improve lol FP progrm No tion in either ommunity tion in Community to estlish or improve lol FP progrm 529

3 ssessing the Ourrene of Unommon Events Not Reommene for Rpi Surveys Neontl Tetnus Community Smple Size for Differene etween Two Proportions Estimtes of Neontl Tetnus in Communities n B CONFIDENCE INTERVLS OF PROPORTION DIFFERENCE 9% 99% Upper CL Point Estimte eths per 1, live irths Estimte prior to survey 15 eths per 1, live irths Lower CL p 2 -p 1 +/- [ x se(p 2 -p 1 ) ] p 2 -p 1 +/- [ 2.45 x se(p 2 -p 1 ) ] p 2 -p 1 +/- [ x se(p 2 -p 1 ) ] If no ifferene -- No tion If ifferene -- tion to ress ifferene - inlue these vlue in grph to shre with eision-mkers or genies funing the survey prior to onuting the fiel stuy Smple Size for Differene etween Two Proportions Estimtes of Neontl Tetnus in Communities n B Estimte Confiene Intervl for Differene etween Neontl Tetnus Estimtes (per 1,) Differene (per 1, live irths) 6 Estimte vlue of first proportion.25 4 Estimte vlue of seon proportion Desire level of onfiene.15 2 Numer of lusters 3 verge numer of persons per luster 1-2 Rte of homogeneity Low onverte to vlue tht orrespons to eff % 99% Confiene Level 534 Smple Size for Differene etween Two Proportions Estimtes of Neontl Tetnus in Communities n B Stnr error of ifferene etween.162 <-- Nee to reue stnr error Smple Size n Deision-mking Estimtes of Neontl Tetnus in Communities n B Differene (per 1, live irths) tion Level 6 Smple size for propose luster survey 3 Design effet for luster survey 2. Upper onfiene limit.43 Point estimte.1 Lower onfiene limit -.23 Is smple size equte for stte nee? NO Not equte euse lower onfiene limit is elow % 99% Confiene Level Community 25 neontl eths per 1, live irths The survey instrument with smple size of 3 in eh ommunity is not useful for guiing neessry eision-mking 15 4 Exess in 2 Community in neontl tetnus eths (%) -2-4 tion in Community to reue neontl tetnus mortlity No tion in either ommunity tion in to reue neontl tetnus mortlity 535

4 Other pplitions of Cluster Surveys Mesure knowlege, ttitues n prties (KP) useful for estimting ommon ttriutes of 1-9% goo for ssessing hnges of 15-2 perentge points or more ssumes 3 persons re mesure in eh ommunity or t eh point in time Deriving ontrols for popultion-se se-ontrol stuy useful for unommon iseses or events with ommon exposures H less thn three ntentl re visits OR Cse-ontrol Stuy Women who elivere in pst yer (e) No (u) Deth in pst yer ue to neontl tetnus (ses) 1 x x No (ontrols) Rpi survey Muh less expensive stuy The os of neontl eth ue to tetnus were 4.7 times greter mong women who h less thn three ntentl re visits s ompre to women who h three or more visits Cohort Stuy (not rpi survey) H less thn three ntentl re visits RR + + Women who eliver uring yer (e) No (u) 1,72 2,28 Deth ue to neontl tetnus No 4.7 1,642 2, ,9 1,72 2,28 4, Very expensive stuy The risk of neontl eth ue to tetnus ws 4.7 times greter mong irths to women who h less thn three ntentl re visits s ompre to irths to women who h three or more visits Cluster Smpling for Rre Events Survey of rre event or isese my e requeste y governmentl or non-governmentl orgniztions Suh surveys ten to e very ostly Exmples HIV prevlene in generl popultion Polio lmeness prevlene in hilren Mesles prevlene in hilren pproh to suh requests fvore y R. R. Frerihs Use Csurvey Progrm Exmple - HIV prevlene survey Intervl estimtion Hypothesis testing Cse-ontrol Stuy Hve unommon event, ourrene or isese (ses) No (ontrols) Disussion with Requesting Offiil Prevlene of HIV Infetion in Rurl Communities Best estimte of true proportion in stuy popultion? History or exposure to risk ftor (e) No (u) p.5 How preise shoul the estimte e?.5%.1 +/-.1% os of exposure mong ses os of exposure mong ontrols Os Rtio How onfient o you nee to e tht erive intervl rkets the true vlue in the stuy popultion? If 3 lusters t 2.45 onfient

5 Enter Vlues in CSurvey Progrm Prevlene of HIV Infetion in Rurl Communities Cluster Smpling for Rre Events Evlution of Intervention (Before vs. fter) Inrese Smple Size to Stisfy Criteri HIV prevlene per 1, 7 Estimte proportion with ttriute.5 6 One-hlf length of onfiene intervl Desire level of onfiene Numer of lusters verge numer of persons per luster Rte of homogeneity Stuent s t for onfiene limit 2.45 sme s SRS - the est tht n e otine % Before ntiipte 6% reution.2% fter The question is, n rpi surveys e use to onut suh n evlution of n intervention progrm with HIV prevlene s the mesure outome? Otin Vlues from CSurvey Progrm Prevlene of HIV Infetion in Rurl Communities The First Step for Hypothesis Testing... Mesuring Chnge in HIV Prevlene Trget stnr error of proportion.489 tul stnr error of proportion.487 Smple size for propose luster survey 21, Design effet for luster survey 1. Upper onfiene limit.6 Point estimte.5 Lower onfiene limit.4 Woul nee to o very lrge survey to fulfill the stte riteri even with n rtifiilly low esign effet ssume t first tht you wnt to mesure 1 persons per luster, s in typil rpi survey. How mny lusters woul e neee? Clultion metho: ( ) Test the propose smple size ( ) Clulte the minimum # of lusters ( ) Clulte the verge # of persons per luster Is smple size equte for stte nee? YES But just rely! If isussion ontinues... Prevlene of HIV Infetion in Rurl Communities Enter Estimtes into CSurvey Progrm Mesuring Chnge in HIV Prevlene Moify the intrlss orreltion oeffiient (roh) to reflet the meium homogeneity tht ours when stuying n infetious isese tht is trnsmitte from one person to nother. repet the new lultions with CSurvey to omply with the stte riteri. Print, sign n te the gree upon smple size estimtion. Estimte vlue of first proportion Estimte vlue of seon proportion Desire level of onfiene verge numer of persons per luster Rte of homogeneity ssume roh equls, s if oing simple rnom smple. This is the optimum ut not neessrily relisti onition for survey 547

6 Otin Vlues from CSurvey Progrm Stnr error of ifferene etween Smple size for propose luster survey Upper onfiene limit Point estimte Mesuring Chnge in HIV Prevlene Minimum numer of lusters , Design effet for luster survey Stuent s t for onfiene limit The esign effet of 1. is not relisti, given tht you re mesuring n infetious isese. Estimte Confiene Intervl for Differene etween HIV Prevlene Estimtes (per 1,) Differene (per 1, popultion) Even with two suh lrge surveys (i.e., 12,8 sujets in eh), the onfiene intervl for the ifferene woul e very wie Lower onfiene limit. -2 9% 99% 548 Confiene Level 551 Repet the Proess with More Relisti Vlues Mesuring Chnge in HIV Prevlene Pst Reing ssignment (Ref. 4) To e Revisite n Disusse Next Estimte vlue of first proportion Estimte vlue of seon proportion Desire level of onfiene verge numer of persons per luster Rte of homogeneity Meium nee to gretly inrese to get equte smple size More relisti -- equivlent to esign effet of Downlo from: Otin Vlues from CSurvey Progrm Mesuring Chnge in HIV Prevlene Minimum numer of lusters Stnr error of ifferene etween Smple size for propose luster survey Design effet for luster survey Upper onfiene limit Point estimte Lower onfiene limit , Stuent s t for onfiene limit Being prtil results in gret inrese in the esign effet n the nee for mny more stuy sujets. 55