T stat vs Z stat

[AlexMo]

Reading the wiley study guide, they present a case in example 1 where they used t stat vs z stat because they only have sample variance, however, since n> 30 wouldnt you use z score regardlesss?

Also,

Y= GMAT Score

X1= # hrs student prepares for test

X2= studnets undergrad GPA

when y = 260.54 + 2.134x1 why would x2 not be constant??

and when y= 231.35 + 1.10X1 + 68.33 X2 the gmat score would increase by 1.103 points holding college gpa constant ?

[Harrogath]

@AlexMo

Reading the wiley study guide, they present a case in example 1 where they used t stat vs z stat because they only have sample variance, however, since n> 30 wouldnt you use z score regardlesss?

It is not adviced. You, as researcher, don’t want to overstate or understate conclusions. So, since only a sample is available, T statistic must be used unless n is very large (80%+ of population)

@AlexMo

Also,

and when y= 231.35 + 1.10X1 + 68.33 X2 the gmat score would increase by 1.103 points holding college gpa constant ?

Yes. Each extra hour the student invests on the GMAT preparation her score, on average, increases in 1.103 points holding undergraduate GPA constant.