# Selecting z-value from table

I am understanding the below points:

I am not able to understand why we select z-points as negative from table at the end? Should we always consider z-point with negative value?

The question is seeking for the probability $$p=P(Z>1.98\cup Z<-1.98)=P(Z>1.98) + P(Z<-1.98)$$
And, the Z-table linked is most probably the CDF of standard normal RV, i.e. $$F_Z(z)=P(Z\leq z)$$. Since this is symmetric with respect to origin, you have $$P(Z>1.98) = P(Z\leq -1.98)$$. Since the mentioned z-table gives us the probabilities for any $$\leq z$$, you use the negative value and multiply by $$2$$: $$p=2P(Z<-1.98)$$
Or, you could've used $$P(Z\leq 1.98)=p_z$$ and calculate $$p=2(1-p_z)$$
• For driving home the point about the "easiest way" (near the end of the quoted material in the question), it might help to contrast this solution with the one that looks up the value $\Pr(Z\le 1.98)$ in the table.