The Standard Normal Distribution

Notice that the x axis is standard scores, also called z scores. This means that the distribution has a population mean of zero, and a population standard deviation of 1.

The most significant thing about the normal distribution is that predictable proportions of cases occur in specific regions of the curve. 50%

Notice that: (1) 34.13% of the scores lie between the mean and 1 sd above the mean, (2) 13.6% of the scores lie between 1 sd above the mean and 2 sds above the mean, (3) 2.14% of the scores lie between 2 sds above the mean and 3 sds above the mean, and (4) 0.1% of the scores in the entire region above 3 sds. The curve is symmetrical, so the area betw 0 and -1 = 34.13%, -1 to -2 = 13.6%, etc. Also, 50% of the cases are above 0, 50% below.

IQ Percentile Problem 1: IQ:  =100,  =15. Convert an IQ score of 115 into a percentile, using the standard normal distribution. Step 1: Convert IQ=115 into a z score: z = x i -  /  5; z = +1.0 (1 sd above the mean) IQ=115, z = 1.0

Step 2: Calculate the area under the curve for all scores below z=1 (percentile=% of scores falling below a score). Area under the curve below z=1.0: = We get this by adding (the area between the mean and z=1) to (50% is the area under the curve for values less than zero; i.e., the entire left side of the bell curve). So, an IQ score of 115 (z=1.0) has a percentile score of IQ=115, z = +1.0 The part with the slanty lines repre- sents the portion of the distribution we’re looking for.

IQ Percentile Problem 2: IQ:  =100,  =15. Convert an IQ score of 85 into a percentile, using the standard normal distribution. Step 1: Convert IQ=85 into a z score: z = x i -  /  5; z = -1.0 (1 sd below the mean) IQ=85, z = -1.0

Step 2: Calculate the area under the curve for all scores below z=-1. Area under the curve values below z=-1.0: = We get this by subtracting (the area between the mean and z=-1) from (50% is the total area under the curve for values less than zero; i.e., the entire left side of the bell curve). So, an IQ score of 85 (z=-1.0) has a percentile score of IQ=85, z = -1.0

z = x i -  /  5 z = -1.0 Area under the curve for z=-1.0: = We get this by subtracting from (50.00 is the total area under the curve for values less than zero; i.e., the entire left side of the bell curve.) We therefore want the 50% minus the area between zero and -1.0). So, an IQ score of 85 has a percentile score of IQ=85, z=-1.0