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A standardized value A number of standard deviations a given value, x, is above or below the mean z = (score (x) – mean)/s (standard deviation) A positive z-score means the value lies above the mean A negative z-score means the value lies below the mean Round to 2 decimals The mean for IQ scores is 50 with a standard deviation of 5 with a normal distribution What is the probability that scores will be between 45 and 55? Calculate z-score first Use the Normal Distribution (z) statistical table Score (x) = 45 Mean = 50 s=5 z = (score (x) – mean)/s (standard deviation) z = (45 - 50)/5 (standard deviation) o o = -5/5, = -1 Wait! There’s a second value to consider. Score (x) = 55 Mean = 50 s=5 z = (score (x) – mean)/s (standard deviation) z = (55 - 50)/5 (standard deviation) o = 5/5, = 1 Using the normal distribution (z) statistical table: Determine the area from the mean: 1 s up (mean to z) = .3413 1 s down (mean to z) = .3413 Add the 2 values together .3413 + .3413 = .6826 * 100% = 68.26% So, the probability that a score will be between 45 and 55 is 68.26%! The mean for IQ scores is 50 with a standard deviation of 5 with a normal distribution What is the probability that an IQ score will be between 55 and 60? Calculate z-score first Use the Normal Distribution (z) statistical table Score (x) = 55 Mean = 50 s=5 z = (score (x) – mean)/s (standard deviation) z = (55 - 50)/5 (standard deviation) o o = 5/5, = 1 Wait! There’s a second value to consider. Score (x) = 60 Mean = 50 s=5 z = (score (x) – mean)/s (standard deviation) z = (60 - 50)/5 (standard deviation) o = 10/5, = 2 Using the normal distribution (z) statistical table: Determine the area from the mean: 1 s up (mean to z) = .3413 2 s up (mean to z) = .4772 Subtract the 2 values (because we only want the distance from 1z to 2z, not the mean to 2z) .4772 - .3413 = .1359 * 100% = 13.59% So, the probability that an IQ score will be between 55 and 60 is 13.59%! The mean for IQ scores is 100 with a standard deviation of 15 with a normal distribution What percentage of scores will lie below 100? Calculate z-score first Use the Normal Distribution (z) statistical table Score (x) = 100 Mean = 100 s = 15 z = (score (x) – mean)/s (standard deviation) z = (100 - 100)/15 (standard deviation) o = 0/15 = 0 Using the normal distribution (z) statistical table: Determine the area from the mean: 0 s down (larger portion) = .5000 So, the probability that a student’s IQ score will be below 100 is 50%. The mean for IQ scores is 100 with a standard deviation of 15 with a normal distribution What percentage of scores will lie below 115? Calculate z-score first Use the Normal Distribution (z) statistical table Score (x) = 115 Mean = 100 s = 15 z = (score (x) – mean)/s (standard deviation) z = (115 - 100)/15 (standard deviation) o = 15/15 = 1 Using the normal distribution (z) statistical table: Determine the area from the mean: 1 s up (larger portion) = .8413 So, the percentage of scores that will be below 115 is 84.13% The mean for IQ scores is 100 with a standard deviation of 15 with a normal distribution What percentage of scores will lie above 115? Calculate z-score first Use the Normal Distribution (z) statistical table Score (x) = 115 Mean = 100 s = 15 z = (score (x) – mean)/s (standard deviation) z = (115 - 100)/15 (standard deviation) o = 15/15 = 1 Using the normal distribution (z) statistical table: Determine the area from the mean: 1 s up (smaller portion) = .1587 So, the probability that a student’s IQ score will be above 115 is 15.87% Note: this area of 15.87% plus the area of scores below 115, 84.13%, equal 100%.