How do you find the critical value of z alpha /2 that corresponds to the given confidence level of 87%?

Critical value is 1.51

The confidence interval is 87%. It is the same as 0.87. ##(.87/2)## = 0.435

Consult the Area Under Normal curve table. It is available in all the statistics book.

Look for a value in the probability area that is very close to 0.435. It is 0.4345 in the table. Find the corresponding z - value .

In the picture given below, follow the arrow mark.