# Why does excel use 2 opposite ways for probability functions

#### vasilist

##### New Member
Hello,

i am taking a grad level statistics, out of 5-6 years of an intro statistics classes and something got me confused.
just functions are based in the a, some in the 1-a in excel. Let me explain

If i want to find the Z critical, for two tailed testing, all i have to do is =normsinv(0,95) and if the absolute value of Z is higher than that, i can reject hypothesis 0.
If i wanna do the similar activity with the t student, or F, i need to type =tinv(0,05, degrees of freedom) or = finv(0,05, df)

Why do we insert the complementary probability and not the direct? Am i the only one thinking this is inconsistent.

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#### Buckeye

##### Member
The t distribution is bell shaped and symmetric. So, if you type in 0.95 with the appropriate df's you'll get the same critical value with an opposite sign. The F distribution is from 0 to infinity. So, what you input in the function will change depending on the test.

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