Posted by
Dominic Lusinchi on
Feb 17, 2007; 7:54am
URL: http://spssx-discussion.165.s1.nabble.com/Unequal-size-groups-tp1073919p1073920.html
Dear colleague,
A general remark: it is not a good idea to exclude "outliers" - unless you
have some very solid reasons to do so.
You can use the MW test for any variable that is measured at least at the
ordinal level - so you can certainly use it for an interval or ratio level
variable.
You can use the MW test when the two groups are of unequal size.
However, as I understand you situation, you have more than one independent
variable, or perhaps covariate. It would seem that a factorial approach
would be more appropriate: the ratio is the dependent variable, the two
weight groups the independent variable, and age and gender as the covariates
(?).
Good luck.
Dominic Lusinchi
Statistician
Far West Research
Statistical Consulting
San Francisco, California
415-664-3032
www.farwestresearch.com
-----Original Message-----
From: Triantafillos Pliakas [mailto:
[hidden email]]
Sent: Friday, February 16, 2007 2:40 PM
Subject: Unequal size groups
Dear List,
I am doing analysis on some data I have on children
aged from birth to 10 years. I have classified
children by age group (in one year intervals), gender
and also by the presence of obesity (Initially I had
three groups -normal weight, overweight and obese- but
due to the small number of overweight and obese
children I grouped them all together in order to
acquire more statistical power; So now I have two
groups - normal weight and overweight-).
I want to assess the differences in a continuous
variable (waist to height ratio) between normal weight
and overweight children, by gender and age group. I
have explored the data using the Kolmogorov-Smirnof
test and Shapiro-Wilk test, excluded potential
outliers and found that in most cases (except only one
age group) the above tests were significant. So
instead of using a parametric test (independent t
test) or even try to transform the data
logarithmically I decided to use the non parapetric
test Mann-Whitney. My questions are:
1) Can I use the Mann-Whitney test even though my test
variable (waist to height ratio) is not ordinal?
2) Even if I can use Mann-Whitney test (or even if I
need to carry out another test) the thing is that I
still have a very small number of overweight children
(even after grouping overweight and obese children).
The number of normal weight children range from 226 to
398 whereas for overweight range from 13 to 44 between
the different age groups(For example I have 320 normal
weight boys and 34 overweight ones in age group 6-7
years and in age group 8-9 years I have 398 normal
weight girls and 13 overweight ones). Does this
underestimate or distort any statistical differences
found? (I assume it does) And if so is there a better
statistical test to carry out to minimize this or at
least to address this more appropriately?
Thank you in advance
Triantafyllos
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