I need some help for my thesis about t-testing different data sets

#1
Hello everybody,

I'm conducting research on price dispersion to show that offline price dispersion is significantly smaller than online price disperison. I have up until today, monitored and collected over 1500 prices in excel on several homogeneous products in dual-channel/online/brick-and-mortar stores.

I have showed the price dispersion using standard deviation, variance, Price percentage difference and coefficient of variance. When i showed him the results he said that i needed to show it was significantly higher/lower and that I should perform a test in excel.

My question is: What test do I need here, excel has three different T-test a Z-test and F-test and I have absolutely no clue how I can compare these:

What I have done now is averaged the prices on the same days for online shops, and averaged the prices for the same goods on the same days for brick&mortar shops and performed a two sided t-test with uneven variances (since it are different shops). Could this be right and how could i conclude that indeed the price dispersion is lower/higher?

I really need some help, it would be so much appreciated. Please let me know,

Kind regards,

Friso
 

EdGr

New Member
#2
Unpaired t-tests will compare the average price online and offline, not the dispersion. Seems like you need a test for equality of variances. Most statistical packages will run this test for you, but I'm not sure Excel will. The formulas are not too difficult, however.

One trick, however -- are the dispersions heavily affected by a few outliers? Comparing dispersions with non-normal distributions is more difficult. One simple approach would be to compare using the equality of variance F test, then remove a pre-determined number of high and low values from BOTH samples (say 10 top and bottom from each -- or a constant % like 1% if the counts were different). Rerun.
 
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