- Robert J Miller added an answer:How can I compare linear relationships?
- Bruce E Oddson added an answer:Does anyone have suggestions for reporting a robust ANCOVA?
- Scott Taylor Barrett added an answer:How do you conduct a mixed-factors ANCOVA with a time-dependent covariate in R?
- Dasapta Erwin Irawan added an answer:How can we determining significance of independent variables in logistics regression?
- Javier Miguelena added an answer:ANCOVA or repeated measures ANOVA?
- Andrew Ekstrom added an answer:Any suggestion about using ANCOVA with repeated measures?
- Roshanak Soltani added an answer:When can I use ANCOVA?
- Roshanak Soltani added an answer:What is the exact name of my test in spss?
- Bruce Weaver added an answer:Can one use multiple logistic regression to estimate possible confounding effect?
- Emir Veledar added an answer:How can you compare two groups that are initially distinct in two moments of time?
- Simona Katholnig asked a question:What does it mean if a covariate turns the effect of an IV on DV from significant to insignificant in an ANCOVA?

We have measured simple relationships between the size of different species and their weight (biomass). They are linear regressions (sometimes semilog). The type of question I would like to answer is, for example, if I have 4 species of snail, each with a separate linear equation representing the relationship between size and biomass, are those relationships significantly different, or does one relationship suffice for all my snail species?

I'm not sure how to do this comparison. Each species often has different sample sizes. I thought about just doing the separate relationships, then pooling all samples and comparing the resulting slope to the species-wise relationships with t tests. However I'm worried that species with more samples will bias the results. I could randomly remove samples from those species to equalize them. Another alternative might be to do an ancova with species put in as a dummy variable and look for interactions with species as a test of parallelism. Does that seem reasonable? It seems like a good idea to me because it will also be a test of whether the intercepts are the same.