Greeting
im a bit confused of how we select the best statistical method to start my data analysis ? When can use ANCOVa or ANOVA? And how i can do the ANCOVA by prism ?thanks for replying in advance
One typically would use an ANCOVA when one has a continuous independent variable, along with the categorical independent variable. For example, say you are studying the effect of a Drug upon recovery of patients. With an ANOVA you can study the recovery with or without the Drug. If, in addition, you have information about say about the Age of your patients, you can run an ANCOVA to study the recovery patients "controling for" Age.
ANCOVA basically looks at the effect of covariates on the results of a designed experiment. The classic paper was by R A Fisher on the effect of rainfall on wheat yields. Rainfall was the covariate because Fisher couldn't control that in his experimental design. You can do both ANOVA and ANCOVA with any regression software.. Details can be found by repeating the attached search. Best wishes David Booth
Brief answer:
-You do ANOVA if you are interested to compare the mean of outcome across multiple categories of the factor variable.
- Sometimes you may also have a mix of continuous covariate and factor variables, in that case, ANOVA is no longer meaningful, rather you should apply ANCOVA .
In an pre/posttest approach (e.g. in medical research), an ANCOVA is a regression of the the post-test measure upon the pre-test measure and the treatment group variable. The ANCOVA has the advantage over a "change score" approach when baseline outcome measureses are available. Yet, in the case of an ANCOVA, the covariates (pre- and post-mesures) are assumed to be unaffected by treatment. (This is the homogeneity of regressions slopes assumption.)
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