Closed BiomechCorey closed 3 years ago
Hello Corey, thank you for this question.
Yes, SPM as a general methodology can handle different-sized groups, but unfortunately the software package spm1d currently supports arbitrary group sizes for only two designs:
From your description it sounds like this is a one-way ANOVA, so I think you'll be able to use spm1d.
This won't violate group size assumptions, because neither equal group sizes nor equal group variance is assumed.
Todd
Brilliant! Thanks for the super quick response, Todd. Your expertise is much appreciated.
Hi again, Todd!
One further clarification on the violation of assumptions. Is it ok to treat individual legs as an independent observation? Most studies I have read on SPM don't have this sort of nuance to use as a guide.
For example, with our dataset, we want to treat legs independently because the kinematic gait pattern is different for each leg. So, we have one patient (001) but two series of data (left and right) for which the left is classed as A and the right is classed as B.
Patient Group
001_Left A
001_Right B
002_Left C
002_Right A
Ultimately, we want to know if the groups are different from each other across the gait cycle and where - which is why SPM so powerful.
Many thanks again,
Corey
Hi Corey,
Is it ok to treat individual legs as an independent observation?
Strictly speaking: no, because contralateral limbs belong to the same individual and are therefore not independent by definition. However, this non-independence problem likely becomes smaller as sample size increases.
Here is a slightly longer answer:
When limbs are not of explicit empirical interest (e.g. injured vs. uninjured), there are a number of options:
As far as I know, (1) is relatively common in the literature, and the other options are not common.
I personally prefer (2), because:
(3) and (4) are probably non-ideal because shuffling time around arbitrarily may introduce a variety of problems like dynamic inconsistency.
So I'm not sure what the best approach is, and perhaps it would be best to try all of the options to understand how different approaches affect the final results.
Todd
Hi there,
We are a clinical gait service in Australia and am very keen to use SPM on a project with kinematic data. We have a particular research question that asks if there is a difference between 5 groups of gait cycle kinematics. The question we have is, do we violate any assumptions if the groups are of different size? For example 1) n=10; 2) n = 5; 3) n= 25.
Many thanks,
Corey