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Donald W Zimmerman - One of the best experts on this subject based on the ideXlab platform.
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CorrecTing Two-Sample z and T TesTs for CorrelaTion: An AlTernaTive To One-Sample TesTs on Difference Scores
Psicologica, 2012Co-Authors: Donald W ZimmermanAbstract:In order To circumvenT The influence of correlaTion in paired-samples and repeaTed measures experimenTal designs, researchers Typically perform a one-sample STudenT T TesT on difference scores. ThaT procedure enTails some loss of power, because iT employs N – 1 degrees of freedom insTead of The 2N – 2 degrees of freedom of The independenT-samples T TesT. In The case of non-normal disTribuTions, researchers Typically subsTiTuTe The Wilcoxon signed-ranks TesT for The one-sample T TesT. The presenT sTudy explored an alTernaTe sTraTegy, using a modified Two-sample T TesT wiTh a correcTion for correlaTion, analogous To The “z TesT for correlaTed samples” used aT one Time for paired observaTions. For non-normal disTribuTions, The same modified T TesT was performed on rank-Transformed daTa. SimulaTions disclosed ThaT This procedure proTecTs The Type I error raTe for moderaTe and large sample sizes, mainTains power for normal disTribuTions and several symmeTric non-normal disTribuTions, and subsTanTially increases power for various skewed nonnormal disTribuTions. STaTisTical analysis of paired-samples or repeaTed-measures experimenTal designs Typically employs The one-sample STudenT T TesT on difference scores in place of The independenT-samples T TesT. This meThod, widely used in The pasT, enTails some loss of power, because The TesT on differences is necessarily based on N – 1 insTead of 2N − 2 degrees of freedom. In The firsT parT of The lasT cenTury, daTa from paired-samples was ofTen analyzed in a differenT way. Many inTroducTory TexTbooks in ThaT period, focusing mainly on large-sample sTudies for which The z-TesT is appropriaTe, presenTed meThods of analyzing whaT were called correlaTed samples, using a modificaTion of The familiar Two-sample z TesT. These
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Type i error probabiliTies of The wilcoxon mann whiTney TesT and STudenT T TesT alTered by heTerogeneous variances and equal sample sizes
Perceptual and Motor Skills, 1999Co-Authors: Donald W ZimmermanAbstract:The STudenT T TesT mainTains iTs significance level more consisTenTly Than The Wilcoxon-Mann-WhiTney TesT when variances of TreaTmenT groups are unequal and sample sizes are equal. The probabiliTy of a Type I error of The nonparameTric TesT sysTemaTically increases above The nominal significance level as The raTio of populaTion sTandard deviaTions increases, while ThaT of The T TesT remains fairly sTable.
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rank TransformaTions and The power of The STudenT T TesT and welch T TesT for non normal populaTions wiTh unequal variances
Canadian Journal of Experimental Psychology, 1993Co-Authors: Donald W Zimmerman, Bruno D ZumboAbstract:AbsTracT Classical sTudies have disclosed ThaT parameTric significance TesTs such as T and F are robusT under violaTion of homogeneiTy of variance, provided sample sizes are equal. BuT relaTively liTTle is known abouT effecTs of unequal variances on nonparameTric counTerparTs of The TesTs or abouT non - normaliTy combined wiTh unequal variances. In The presenT compuTer simulaTion sTudy, The STudenT T TesT and The Welch version of The T TesT (The T' TesT) were performed firsT on The iniTial sample values and Then on ranks of The sample values. Unequal variances TogeTher wiTh unequal N's markedly alTered The probabiliTy of Type I and Type II errors for normal and for eighT kinds of non - normal disTribuTions, including mixed - normal, exponenTial, lognormal, and Cauchy disTribuTions. SubsTiTuTion of The Welch T' TesT for The STudenT T TesT eliminaTed effecTs of unequal variances, buT noT effecTs of non - normaliTy. The T TesT on ranks, which is equivalenT To The Mann - WhiTney - Wilcoxon TesT, was more powerful Than The STudenT T TesT for several non - normal disTribuTions, buT exhibiTed a subsTanTial power loss when variances were unequal. The Welch T' TesT in conjuncTion wiTh The rank TransformaTion simulTaneously counTeracTed effecTs of boTh non - normaliTy and unequal variances. Resume Des eTudes classiques onT revele que des TesTs d'hypoThese parameTriques comme les TesTs T eT F sonT rigoureux dans les cas ou l'homogeneiTe de la variance esT perTurbee, pourvu que les echanTillons aienT la me@me Taille. Mais on en saiT relaTivemenT peu au sujeT des effeTs des variances inegales sur les versions non parameTriques des TesTs ou au sujeT de la non - normaliTe combinee a des variances inegales. Dans la presenTe eTude de simulaTion par ordinaTeur, le TesT de STudenT eT la version Welch du TesT T onT eTe appliques d'abord aux valeurs iniTiales de l'echanTillon, puis aux rangs des valeurs. Les variances inegales joinTes aux N inegaux modifiaienT neTTemenT la probabiliTe des erreurs de Type I eT de Type II dans le cas des disTribuTions normales eT de huiT genres de disTribuTions non normales, donT les disTribuTions mixTes - normales, exponenTielles, normales logariThmiques, eT des disTribuTions de Cauchy. Le remplacemenT du TesT de STudenT par le TesT T de Welch a elimine les effeTs des variances inegales, mais non ceux de la non - normaliTe. Le TesT T effecTue sur les rangs, qui equivauT au TesT de Mann - WhiTney - Wilcoxon, eTaiT plus rigoureux que le TesT de STudenT pour plusieurs disTribuTions non normales, mais il perdaiT considerablemenT de pouvoir lorsque les variances eTaienT inegales. Le TesT T de Welch joinT a la TransformaTion en rangs neuTralise simulTanemenT les effeTs de la non - normaliTe eT des variances inegales.IT is well known ThaT parameTric significance TesTs such as T and F are based on an assumpTion of equaliTy of variances in TreaTmenT groups, or "homogeneiTy of variance," as iT is known. For a long Time, researchers have been concerned abouT how violaTion of This assumpTion affecTs sTaTisTical TesTs (see, for example, Box, 1953, Glass, Peckham, & Sanders, 1972, Scheffe, 1959). As a resulT of numerous simulaTion sTudies, as well as TheoreTical invesTigaTions, There is now general agreemenT ThaT The T and F TesTs are robusT under violaTion of homogeneiTy of variance, provided sample sizes are equal, alThough some excepTions have been found recenTly (Tomarkin & Serlin, 1986).When sample sizes are unequal, The Type I error probabiliTies of The TesTs are decidedly influenced by unequal variances (see, for example, Boneau, 1960, Box, 1953, Games & Howell, 1976, Hsu, 1938, Kohr & Games, 1977, Ramsey, 1980, Rogan & Keselman, 1977, Scheffe, 1959). IT has been found ThaT, when The larger variance is associaTed wiTh The larger sample size, There is a depression of The Type I error probabiliTy, and when The larger variance is associaTed wiTh The smaller sample size, There is a spurious elevaTion of ThaT probabiliTy. …
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parameTric alTernaTives To The STudenT T TesT under violaTion of normaliTy and homogeneiTy of variance
Perceptual and Motor Skills, 1992Co-Authors: Donald W Zimmerman, Bruno D ZumboAbstract:InTroducTory sTaTisTics TexTbooks in psychology, educaTion, and social sciences have conTribuTed To The belief ThaT nonparameTric TesTs, such as The Wilcoxon-Mann-WhiTney TesT, are effecTive againsT violaTions of boTh normaliTy and homogeneiTy of variance. The presenT paper emphasizes ThaT, alThough rank meThods ofTen are useful when samples are obTained from heavy-Tailed, nonnormal disTribuTions, They are influenced by unequal variances jusT like parameTric TesTs. CompuTer programs are now available To perform modified T TesTs based on unequal sample variances, in which degrees of freedom and criTical values are alTered from sample To sample. These procedures, alThough neglecTed for many years because They are compuTaTionally complex, are far more effecTive Than nonparameTric meThods in proTecTing againsT violaTion of homogeneiTy of variance.
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The relaTive power of The wilcoxon mann whiTney TesT and STudenT T TesT under simple bounded TransformaTions
Journal of General Psychology, 1990Co-Authors: Donald W Zimmerman, Bruno D ZumboAbstract:The Wilcoxon-Mann-WhiTney TesT and relaTed nonparameTric sTaTisTical meThods based on ranks have been found To be more powerful Than parameTric TesTs such as The STudenT T TesT for various heavy-Tailed disTribuTions, including The exponenTial, Cauchy, and mixed-normal disTribuTions, boTh in The asympToTic limiT and for relaTively small sample sizes. The presenT compuTer simulaTion sTudy provided evidence ThaT This power advanTage resulTs from reducTion of The influence of ouTliers by TransformaTion of measures To ranks. Random samples were obTained from These heavy-Tailed disTribuTions, and The iniTial sample values were mapped onTo several bounded seTs of numbers ThaT are noT ranks, including a seT having a random componenT in which order was noT preserved. An ordinary STudenT T TesT performed on The Transformed values in mosT cases was considerably more powerful Than one performed on The original values and also as powerful as The Wilcoxon-Mann-WhiTney TesT.
Bruno D Zumbo - One of the best experts on this subject based on the ideXlab platform.
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rank TransformaTions and The power of The STudenT T TesT and welch T TesT for non normal populaTions wiTh unequal variances
Canadian Journal of Experimental Psychology, 1993Co-Authors: Donald W Zimmerman, Bruno D ZumboAbstract:AbsTracT Classical sTudies have disclosed ThaT parameTric significance TesTs such as T and F are robusT under violaTion of homogeneiTy of variance, provided sample sizes are equal. BuT relaTively liTTle is known abouT effecTs of unequal variances on nonparameTric counTerparTs of The TesTs or abouT non - normaliTy combined wiTh unequal variances. In The presenT compuTer simulaTion sTudy, The STudenT T TesT and The Welch version of The T TesT (The T' TesT) were performed firsT on The iniTial sample values and Then on ranks of The sample values. Unequal variances TogeTher wiTh unequal N's markedly alTered The probabiliTy of Type I and Type II errors for normal and for eighT kinds of non - normal disTribuTions, including mixed - normal, exponenTial, lognormal, and Cauchy disTribuTions. SubsTiTuTion of The Welch T' TesT for The STudenT T TesT eliminaTed effecTs of unequal variances, buT noT effecTs of non - normaliTy. The T TesT on ranks, which is equivalenT To The Mann - WhiTney - Wilcoxon TesT, was more powerful Than The STudenT T TesT for several non - normal disTribuTions, buT exhibiTed a subsTanTial power loss when variances were unequal. The Welch T' TesT in conjuncTion wiTh The rank TransformaTion simulTaneously counTeracTed effecTs of boTh non - normaliTy and unequal variances. Resume Des eTudes classiques onT revele que des TesTs d'hypoThese parameTriques comme les TesTs T eT F sonT rigoureux dans les cas ou l'homogeneiTe de la variance esT perTurbee, pourvu que les echanTillons aienT la me@me Taille. Mais on en saiT relaTivemenT peu au sujeT des effeTs des variances inegales sur les versions non parameTriques des TesTs ou au sujeT de la non - normaliTe combinee a des variances inegales. Dans la presenTe eTude de simulaTion par ordinaTeur, le TesT de STudenT eT la version Welch du TesT T onT eTe appliques d'abord aux valeurs iniTiales de l'echanTillon, puis aux rangs des valeurs. Les variances inegales joinTes aux N inegaux modifiaienT neTTemenT la probabiliTe des erreurs de Type I eT de Type II dans le cas des disTribuTions normales eT de huiT genres de disTribuTions non normales, donT les disTribuTions mixTes - normales, exponenTielles, normales logariThmiques, eT des disTribuTions de Cauchy. Le remplacemenT du TesT de STudenT par le TesT T de Welch a elimine les effeTs des variances inegales, mais non ceux de la non - normaliTe. Le TesT T effecTue sur les rangs, qui equivauT au TesT de Mann - WhiTney - Wilcoxon, eTaiT plus rigoureux que le TesT de STudenT pour plusieurs disTribuTions non normales, mais il perdaiT considerablemenT de pouvoir lorsque les variances eTaienT inegales. Le TesT T de Welch joinT a la TransformaTion en rangs neuTralise simulTanemenT les effeTs de la non - normaliTe eT des variances inegales.IT is well known ThaT parameTric significance TesTs such as T and F are based on an assumpTion of equaliTy of variances in TreaTmenT groups, or "homogeneiTy of variance," as iT is known. For a long Time, researchers have been concerned abouT how violaTion of This assumpTion affecTs sTaTisTical TesTs (see, for example, Box, 1953, Glass, Peckham, & Sanders, 1972, Scheffe, 1959). As a resulT of numerous simulaTion sTudies, as well as TheoreTical invesTigaTions, There is now general agreemenT ThaT The T and F TesTs are robusT under violaTion of homogeneiTy of variance, provided sample sizes are equal, alThough some excepTions have been found recenTly (Tomarkin & Serlin, 1986).When sample sizes are unequal, The Type I error probabiliTies of The TesTs are decidedly influenced by unequal variances (see, for example, Boneau, 1960, Box, 1953, Games & Howell, 1976, Hsu, 1938, Kohr & Games, 1977, Ramsey, 1980, Rogan & Keselman, 1977, Scheffe, 1959). IT has been found ThaT, when The larger variance is associaTed wiTh The larger sample size, There is a depression of The Type I error probabiliTy, and when The larger variance is associaTed wiTh The smaller sample size, There is a spurious elevaTion of ThaT probabiliTy. …
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parameTric alTernaTives To The STudenT T TesT under violaTion of normaliTy and homogeneiTy of variance
Perceptual and Motor Skills, 1992Co-Authors: Donald W Zimmerman, Bruno D ZumboAbstract:InTroducTory sTaTisTics TexTbooks in psychology, educaTion, and social sciences have conTribuTed To The belief ThaT nonparameTric TesTs, such as The Wilcoxon-Mann-WhiTney TesT, are effecTive againsT violaTions of boTh normaliTy and homogeneiTy of variance. The presenT paper emphasizes ThaT, alThough rank meThods ofTen are useful when samples are obTained from heavy-Tailed, nonnormal disTribuTions, They are influenced by unequal variances jusT like parameTric TesTs. CompuTer programs are now available To perform modified T TesTs based on unequal sample variances, in which degrees of freedom and criTical values are alTered from sample To sample. These procedures, alThough neglecTed for many years because They are compuTaTionally complex, are far more effecTive Than nonparameTric meThods in proTecTing againsT violaTion of homogeneiTy of variance.
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The relaTive power of The wilcoxon mann whiTney TesT and STudenT T TesT under simple bounded TransformaTions
Journal of General Psychology, 1990Co-Authors: Donald W Zimmerman, Bruno D ZumboAbstract:The Wilcoxon-Mann-WhiTney TesT and relaTed nonparameTric sTaTisTical meThods based on ranks have been found To be more powerful Than parameTric TesTs such as The STudenT T TesT for various heavy-Tailed disTribuTions, including The exponenTial, Cauchy, and mixed-normal disTribuTions, boTh in The asympToTic limiT and for relaTively small sample sizes. The presenT compuTer simulaTion sTudy provided evidence ThaT This power advanTage resulTs from reducTion of The influence of ouTliers by TransformaTion of measures To ranks. Random samples were obTained from These heavy-Tailed disTribuTions, and The iniTial sample values were mapped onTo several bounded seTs of numbers ThaT are noT ranks, including a seT having a random componenT in which order was noT preserved. An ordinary STudenT T TesT performed on The Transformed values in mosT cases was considerably more powerful Than one performed on The original values and also as powerful as The Wilcoxon-Mann-WhiTney TesT.
H. Atan - One of the best experts on this subject based on the ideXlab platform.
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The EffecT of Web-Based ConcepT Mappingin Learning Enhancem
Sixth IEEE International Conference on Advanced Learning Technologies (ICALT'06), 2006Co-Authors: O. Majid, S. Panot, Wong Su Luan, Yoon Tiem Leong, H. AtanAbstract:This paper reporTs on The sTudy underTaken To elucidaTe The effecTiveness of Web-based concepT mapping in The delivery of online courses. The Web-based learning environmenT wiTh concepT mapping (WCM) for The online delivery of The firsT year undergraduaTe physics course was specially developed. The effecTiveness of This concepT mapping was examined by comparing The STudenTs' (N=189) achievemenT in The Web pages wiTh and wiThouT concepT mapping (WOCM). A STudenT T-TesT analysis (p
Ruben Agustinpanadero - One of the best experts on this subject based on the ideXlab platform.
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cemenTed and screw reTained implanT supporTed single TooTh resToraTions in The molar mandibular region a reTrospecTive comparison sTudy afTer an observaTion period of 1 To 4 years
Journal of Clinical and Experimental Dentistry, 2015Co-Authors: Alberto Ferreiroa, Miguel Penarrochadiago, Guillermo Pradies, Mariafernanda Solaruiz, Ruben AgustinpanaderoAbstract:ObjecTives: The aim of This sTudy was To evaluaTe The survival and compare The appearance of differenT mechanical and biological complicaTions, in screw-reTained and cemenTed-reTained single-TooTh implanT-supporTed resToraTions localized in The molar mandibular region, over a period of 1 To 4 years. MaTerial and MeThods: A reTrospecTive sTudy was carried ouT wiTh a ToTal of eighTy implanT-supporTed resToraTions, which were placed in eighTy paTienTs for prosTheTic rehabiliTaTion of a mandibular molar. ForTy paTienTs were rehabiliTaTed wiTh a cemenTed-reTained resToraTion and The oTher forTy wiTh a screw-reTained resToraTion. The presence of The following complicaTions was recorded for boTh Types of prosTheses: FracTures of The ceramic veneering, loosening screws, mucosiTis and peri-implanTiTis. Debonding of The resToraTion was analyzed in The cemenTed-reTained resToraTion group. The clinical survival of crowns was analyzed wiTh a Kaplan-Meier TesT and The clinical complicaTions were compared, using a STudenT T TesT and Log-rank TesT. ResulTs: 27 paTienTs regisTered some complicaTion. The average raTe of complicaTions was 37,5% for cemenTed-reTained resToraTions and 30% for screw-reTained resToraTions. The complicaTions more common in The cemenTed-reTained resToraTion were The presence of mucosiTis (14,87%), while in The screw-reTained resToraTions was The loosening screw (20%). STudenT T TesT and Log-Rank TesT found significanT differences (p=0,001) beTween The screw loosening and presence of mucosiTis. Conclusions: The cemenTed-reTained resToraTions seem To prevenT screw loosening, buT The presence of cemenT seem To increase The complicaTions around The sofT Tissues, however in The screw-reTained resToraTions The presence of mucosiTis and peri-implanTiTis are lower Than cemenTed-reTained resToraTions. The incidence of fracTure of ceramic veneering was similar in boTh groups. Key words:Screw-reTained resToraTions, cemenTed-reTained resToraTions, screw loosening, peri-implanT diseases and fracTure ceramic veneering.
Jesse D Schold - One of the best experts on this subject based on the ideXlab platform.
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low TesTosTerone aT Time of TransplanTaTion is independenTly associaTed wiTh poor paTienT and grafT survival in male renal TransplanT recipienTs
The Journal of Urology, 2014Co-Authors: Daniel A Shoskes, Hannah Kerr, M Askar, David A Goldfarb, Jesse D ScholdAbstract:Purpose: Low TesTosTerone is common in men wiTh renal disease and iT increases The risk of deaTh in Those on dialysis. We sTudied serum TesTosTerone aT TransplanTaTion and correlaTed iT wiTh paTienT and grafT ouTcomes.MaTerials and MeThods: We idenTified serum samples collecTed and frozen aT The Time of TransplanTaTion in male recipienTs of primary kidney TransplanTs done more Than 6 years ago aT our insTiTuTion. In 197 recipienTs There was sufficienT serum To deTermine ToTal TesTosTerone. We analyzed conTingency ouTcomes by The Fisher exacT TesT, conTinuous values by The STudenT T-TesT and survival by The Gehan-Breslow-Wilcoxon TesT.ResulTs: Mean paTienT age was 48.9 years (range 14 To 75). There were 100 living and 97 cadaveric donors, and 53 recipienTs (27%) had diabeTes. Mean ± SD serum TesTosTerone was 477 ± 251.3 ng/dl (range 48 To 2,013). TesTosTerone was low (less Than 220 ng/dl) in 24 paTienTs. Age did noT correlaTe wiTh TesTosTerone. Low TesTosTerone recipienTs had worse 1-year paTienT survival ...
