Rylan is testing 2 different types of nail polish on her fingernails to determine which one lasts longer. she purchases brand a and brand b from the drug store. she then paints her nails with brand a and her sister's nails with brand
b. she compares the nails polish at the end of the week and determines that brand b last longer because there fewer chips than brand
a. why is her conclusion not valid?

If you want to compare the quality of something, you should make sure that all the conditions were similar. This means that if you want to compare the quality of nail polish, you should expose both kinds of nail polish to the same tests. However, here we don't know whether Rylan and her sister did the same things this week: for example if one of them washed more dishes, her nail polish would have a "harder" test - the test did not make sure that both products experienced the same conditions!