I have three groups, one control, one having high fat diet and one having high fat diet+treatment and i want to measure the concentration of glucose over a period of time in these groups. Should I use Two-way ANOVA or ANCOVA? Please help.
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$\begingroup$ Do you mean the concentration of glucose is your response variable and it will be measured several times from single individual? $\endgroup$– user158565Commented Nov 25, 2018 at 16:12
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First, if you have multiple measures for each subject (person? animal?) over time then you should not do ANOVA or ANCOVA as they both assume that the errors are independent and that will not be the case with multiple measures.
Second, ANOVA and ANCOVA are the same model (and also the same as OLS regression). In matrix terms, both are $Y = XB + e$ where Y is a vector of responses, X is a matrix of independent variables (e.g. group), B is a vector of parameters to be estimated and e is error. However, if you have only one measure per time point, then it looks like you have only one independent variable (group) so one-way ANOVA would be fine (as would regression, which is equivalent) - provided that it meets the other assumptions.
answered Nov 25, 2018 at 10:36