Normal distribution tables - right or left?

121 Views Asked by At

Are the probabilities in normal distribution tables given typically to the right or left of the $Z$ score?

One such text I am reading says to the right.

However, in my lecturer's exercises, I encountered a problem asking:

If $X \sim N(1,2) $ find $P(X>0)$.

Since $Z \sim \frac{X-\mu}{\sigma}$ then we have

$P(X>0) = P(\frac{X-1}{\sqrt{2}} > \frac{-1}{\sqrt{2}}) = P(Z > \frac{-1}{\sqrt{2}})$

= $1 - \Phi(-\frac{1}{\sqrt{2}})$

Now, if it's to the right, and we want to find $P(Z > \frac{-1}{\sqrt{2}})$

Don't we just need to look up $-\frac{1}{\sqrt{2}}$ in our table? Why would we minus it by one? The table gives us the values to the right of $\frac{-1}{\sqrt{2}}$ already.