ANCOVA when variances are not equal?

Roy99

New Member
#1
I am currently writing a 4th year freshwater biology report analysing data collected on the length and weight of fish from 2 sites, collected over 3 years.

We need run an ANCOVA checking Weight v Length/Site(SPSS, weight=dependent, site=factor, length=covariante).

After checking assumptions of equal variance we had to log transform the data. Variances were checked again yet were still not equal p<0.05.

Any ideas on how we should attempt to analyse this data as litereature seems to rule out the idea of using any standard non-parametric tests.

Thanks

Roy
 

noetsi

Fortran must die
#3
If your sample sizes are relatively equal between levels you might be able to get away with unequal variances. If they are not the situation is much more difficult.

Which I am sure is what Dason was pointing out :)
 

Roy99

New Member
#4
sample sizes are fairly equal, approx 100 and 110 so hopefully running with unequal variances will be ok. Thanks for the help guys.
 
#5
HI all!
I am having the same problem for a project that i am doing (unequal variances: variance for one group is more than 3 times smaller than for the other group), only that sample sizes are quite unequal: 25 vs 75. So i think i definitly cannot do ANCOVA. Does any of you know, if there is a way of still testing a group comparison while accounting for covariets and how i should do it?
Thanks,
Emily
 

HBS

New Member
#6
i guess u can select randomly 25 out of 75 cases, so the the sample size becomes equal. then u can go on doing ANCOVA. U can take significant at p-value of <.001, so that committing typeI error is very less.
 

rogojel

TS Contributor
#7
hi,
I cannot imagine that throwing away information would be of any help, though I admit the idea tempted me as well. You could try some other transformations beside the log, like Box-Cox or Johnson.

reagards