need help on statistic

[dekamii]

sorry i had to do this, don't know where else to ask but BG....lol

The wechsler intelligence scale for children is normally distributed with mean 100 and standard deviation 15.

Q) determine the intelligence score will place a child in the top 5% of all children.

i found the solution
which is: mean + (Z * standard deviation) = 100 + 1.645 * 15

my question is how do you find 1.645 (Z)?
i know how to get Z (assuming that Z means standard unit) but it doesn't give me 1.645....

help!

thanks everybody!

[aurik]

1.645 is obtained by solving the normal cumulative distribution function for p=0.95. That's a bit numerically intensive, so they probably give you a table.

Using the table on wikipedia, (Normal distribution - Wikipedia, the free encyclopedia) you can see that 90% of the probability falls within 1.645~ standard deviations of the mean. The remaining 10% are distributed as 5% above the interval and and 5% below.

Therefore, 5% of the probability falls above 1.645~ stddevs of the mean.