topic Re: Significant Levene's Test for Homogeneity in Statistical Procedures

Significant Levene's Test for Homogeneity

Yughaber — Wed, 31 Mar 2021 20:55:26 GMT

I have a dataset where I want to see if the time of the day (divided into categories of 4 hour intervals) has an effect/makes a difference in the length of stay of visitors. I thought to use one way ANOVA, however, when running the Levene's Test for homogeneity, I got a significant p-value which doesn't meet my assumption of equal variance. Any suggestions of what is the best test to use in this case? Thanks in advance!

Re: Significant Levene's Test for Homogeneity

PGStats — Wed, 31 Mar 2021 21:05:22 GMT

First, check your data for outliers. Otherwise, the Wilcoxon test provided in proc NPAR1WAY is a good alternative.

Re: Significant Levene's Test for Homogeneity

jiltao — Wed, 31 Mar 2021 21:11:50 GMT

You might also consider using PROC MIXED. PROC MIXED allows you to model the unequal variances. For example,

proc mixed;

class hour;

model y=hour;

repeated / group=hour;

run;

This program estimates different variances in y for different groups in HOUR, and the inferences would take into account different variance estimates.

Re: Significant Levene's Test for Homogeneity

Yughaber — Wed, 31 Mar 2021 21:22:58 GMT

take out outliers if there are any?

Re: Significant Levene's Test for Homogeneity

Yughaber — Wed, 31 Mar 2021 21:32:24 GMT

this will only explain the variance right? I can't go further with proving any significant differences between the different time stamped visitor stays

Re: Significant Levene's Test for Homogeneity

PGStats — Wed, 31 Mar 2021 21:32:59 GMT

Take out outliers if they are truly aberrant (i.e. measurement errors or measurements outside the scope of your study). But don't throw them out just for the sake of running a parametric test. Use a robust test like the one I suggested instead.

Re: Significant Levene's Test for Homogeneity

Yughaber — Wed, 31 Mar 2021 21:41:40 GMT

I see, ok that sounds nice. How would I code for the proc NPAR1WAY?

Re: Significant Levene's Test for Homogeneity

PGStats — Wed, 31 Mar 2021 22:15:44 GMT

Simple. For example, does car engine size differ among car models of different origins?

proc npar1way data=sashelp.cars wilcoxon;
class origin;
var enginesize;
run;

Note, the Wilcoxon test is called Kruskal-Wallis when comparing more than 2 categories.

Re: Significant Levene's Test for Homogeneity

Yughaber — Wed, 31 Mar 2021 22:26:52 GMT

awesome! Thanks!!

Re: Significant Levene's Test for Homogeneity

jiltao — Thu, 01 Apr 2021 11:19:21 GMT

Yes, you can use the LSMEANS statement in PROC MIXED to do comparisons.

Re: Significant Levene's Test for Homogeneity

Yughaber — Fri, 02 Apr 2021 19:05:59 GMT

For the median plot in NPAR1WAY, do you know how I can display how many counts/frequencies are above the median and how many are below? The plot only gives a total N and just shows a graph of above and below the median but I need to also know the frequencies above and below...