Importing data into inference app

[Han]

(02-05-2015 08:17 PM)Tim Wessman Wrote:  Z assumes you have a known population standard deviation - not any kind of an estimate. When you are importing for the Z test it will not calculate anything, nor change the entered values, nor modify the existing value. That .2887 is simply the initial default value. The user is responsible for entering the population standard deviation. If the user decides to trust an estimate then that is fine for the user - however the calculator should not spit out an estimate that will be trusted blindly.

If it modified and provided an estimate based on the number of data points then many students would learn an incorrect principle and assume the calculator is "correct" when I am pretty certain it is doing exactly what it should be by definition.


Your given example is essentially a textbook perfect example of when to use a T-test. Saying that the Z test - when is being used incorrectly in this situation unless I am totally misunderstanding everything I thought I knew about stats - should automatically correct and compensate for using the wrong mathematical operation seems to me like saying that 3/4 should actually mean 3*4 and it should have known you mean the multiplication instead.


If you select T-test and import the given data you will get a calculated result that is correct (and the sample standard deviation is automatically calculated like you hope). Doing so using a Z test in nearly all circumstances would be considered the incorrect way to do this calculation.


I suppose that adding a "estimate standard deviation" check box or something could possibly resolve this situation, but I feel it would do so to the detriment of students. n>30 is an old rule and does not always apply. I could add more and more options for the import, but the complexity would far outweigh the benefits.

May I make a recommendation that no default value be placed in the input for the population s.d. That is, the default value should be a blank. A default value of .2887 should not be used for the same reason that any estimated value should not be used.