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z-test is used only in textbooks. Do you mean t-test? -------- Original Message -------- Subject: Re: Help with ANCOVA From: Mubasiru Lamidi <[log in to unmask]> Date: Tue, April 01, 2014 1:07 pm To: [log in to unmask] I agree with permutation test, however if the test was conducted in the same population before and after the application of treatment, test of difference could be applied. If the distribution of the difference is approximately normal, z - test could be used on one hand. On the other hand, if the distribution of the difference violates normality assumption, them go for permutation test or fit regression model and test for normality assumption. It has widely been agreed upon that regression model is more powerful to z - test and this will show true distribution of the data. If the normality assumption is violated, appropriate transformation may be sought and applied appropriately. ******************************************************* Mubasiru Asafe Lamidi Cell#: 403 – 473 – 0642 Home#: 587 – 288 - 9671 -----Original Message----- From: A UK-based worldwide e-mail broadcast system mailing list [mailto:[log in to unmask]] On Behalf Of Phillip Good Sent: Tuesday, April 01, 2014 1:59 PM To: [log in to unmask] Subject: Re: Help with ANCOVA Use a permutation test -------- Original Message -------- Subject: Help with ANCOVA From: Graham Stirrat Clarke <[log in to unmask]> Date: Tue, April 01, 2014 6:17 am To: [log in to unmask] Dear All, A colleague has a before and after treatment dataset to analyse. He has the pre-variable, the post-variable and a treatment variable. He has used ANCOVA. However looking at his data both the pre-variable and post-variable have a far from normal distribution (looks like an exponential, with lots of 0 and 1 values dropping down smoothly to 12 – there are no negative values). I’m not convinced he can use ANCOVA in these circumstances or am I wrong? One thing I’ve noticed is that the difference between pre and post values does follow an approximately normal distribution. Could we analyse the differences instead? Would repeated measures ANOVA or one of its derivatives be acceptable? Any help or advice you could offer would be appreciated. Thank you, Graham Rhif Elusen Gofrestredig 1141565 - Registered Charity No. 1141565 Gall y neges e-bost hon, ac unrhyw atodiadau a anfonwyd gyda hi, gynnwys deunydd cyfrinachol ac wedi eu bwriadu i'w defnyddio'n unig gan y sawl y cawsant eu cyfeirio ato (atynt). Os ydych wedi derbyn y neges e-bost hon trwy gamgymeriad, rhowch wybod i'r anfonwr ar unwaith a dilewch y neges. Os na fwriadwyd anfon y neges atoch chi, rhaid i chi beidio a defnyddio, cadw neu ddatgelu unrhyw wybodaeth a gynhwysir ynddi. Mae unrhyw farn neu safbwynt yn eiddo i'r sawl a'i hanfonodd yn unig ac nid yw o anghenraid yn cynrychioli barn Prifysgol Bangor. Nid yw Prifysgol Bangor yn gwarantu bod y neges e-bost hon neu unrhyw atodiadau yn rhydd rhag firysau neu 100% yn ddiogel. Oni bai fod hyn wedi ei ddatgan yn uniongyrchol yn nhestun yr e-bost, nid bwriad y neges e-bost hon yw ffurfio contract rhwymol - mae rhestr o lofnodwyr awdurdodedig ar gael o Swyddfa Cyllid Prifysgol Bangor. This email and any attachments may contain confidential material and is solely for the use of the intended recipient(s). If you have received this email in error, please notify the sender immediately and delete this email. If you are not the intended recipient(s), you must not use, retain or disclose any information contained in this email. Any views or opinions are solely those of the sender and do not necessarily represent those of Bangor University. Bangor University does not guarantee that this email or any attachments are free from viruses or 100% secure. Unless expressly stated in the body of the text of the email, this email is not intended to form a binding contract - a list of authorised signatories is available from the Bangor University Finance Office. You may leave the list at any time by sending the command SIGNOFF allstat to [log in to unmask], leaving the subject line blank. You may leave the list at any time by sending the command SIGNOFF allstat to [log in to unmask], leaving the subject line blank. You may leave the list at any time by sending the command SIGNOFF allstat to [log in to unmask], leaving the subject line blank. You may leave the list at any time by sending the command SIGNOFF allstat to [log in to unmask], leaving the subject line blank.