# Measures of Position Percentiles Z-scores.

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Measures of Position Percentiles Z-scores

The following represents my results when playing an online sudoku game…at www.websudoku.com.
30 min 0 min

Introduction A student gets a test back with a score of 78 on it.
A 10th-grader scores 46 on the PSAT Writing test Isolated numbers don’t always provide enough information…what we want to know is where we stand.

Where Do I Stand? Let’s make a dotplot of our heights from 58 to 78 inches. How many people in the class have heights less than you? What percent of the dents in the class have heights less than yours? This is your percentile in the distribution of heights

Finishing…. Calculate the mean and standard deviation.
Where does your height fall in relation to the mean: above or below? How many standard deviations above or below the mean is it? This is the z-score for your height.

Let’s discuss What would happen to the class’s height distribution if you converted each data value from inches to centimeters. (2.54cm = 1 in) How would this change of units affect the measures of center, spread, and location (percentile & z-score) that you calculated.

National Center for Health Statistics
Look at Clinical Growth Charts at

Percentiles Value such that r% of the observations in the data set fall at or below that value. If you are at the 75th percentile, then 75% of the students had heights less than yours.

Test scores on last AP Test. Jenny made an 86
Test scores on last AP Test. Jenny made an 86. How did she perform relative to her classmates? 6 7 9 03 Her score was greater than 21 of the 25 observations. Since 21 of the 25, or 84%, of the scores are below hers, Jenny is at the 84th percentile in the class’s test score distribution.

Find the percentiles for the following students….
6 7 9 03 Find the percentiles for the following students…. Mary, who earned a 74. Two students who earned scores of 80.

Cumulative Relative Frequency Table:
Age of First 44 Presidents When They Were Inaugurated Age Frequency Relative frequency Cumulative frequency Cumulative relative frequency 40-44 2 2/44 = 4.5% 2/44 = 4.5% 45-49 7 7/44 = 15.9% 9 9/44 = 20.5% 50-54 13 13/44 = 29.5% 22 22/44 = 50.0% 55-59 12 12/44 = 34% 34 34/44 = 77.3% 60-64 41 41/44 = 93.2% 65-69 3 3/44 = 6.8% 44 44/44 = 100%

Cumulative Relative Frequency Graph:

Interpreting… Why does it get very steep beginning at age 50?
When does it slow down? Why? What percent were inaugurated before age 70? What’s the IQR? Obama was 47….

Describing Location in a Distribution
Interpreting Cumulative Relative Frequency Graphs Use the graph from page 88 to answer the following questions. Was Barack Obama, who was inaugurated at age 47, unusually young? Estimate and interpret the 65th percentile of the distribution Describing Location in a Distribution 65 11 58 47 14

Median Income for US and District of Columbia.
Frequency Relative Cumulative 35 to < 40 1 40 to < 45 10 45 to < 50 14 50 to < 55 12 55 to < 60 5 60 to < 65 6 65 to < 70 3

Graph it: Median Income (\$1000s) Frequency Relative Cumulative
35 to < 40 1 1/51 = 0.020 40 to < 45 10 10/51 = 0.196 11 11/51 = 0.216 45 to < 50 14 14/51 = 0.275 25 25/51 = 0.490 50 to < 55 12 12/51 = 0.236 37 37/51 = 0.725 55 to < 60 5 5/51 = 0.098 42 42/51 = 0.824 60 to < 65 6 6/51 = 0.118 48 48/51 = 0.941 65 to < 70 3 3/51 = 0.059 51 51/51 = 1.000

What is the relationship between percentiles and quartiles?

Z-Score – (standardized score)
It represents the number of deviations from the mean. If it’s positive, then it’s above the mean. If it’s negative, then it’s below the mean. It standardized measurements since it’s in terms of st. deviation.

Discovery: Mean = 90 St. dev = 10 Find z score for 80 95 73

Z-Score Formula

Compare…using z-score.
History Test Mean = 92 St. Dev = 3 My Score = 95 Math Test Mean = 80 St. Dev = 5 My Score = 90

Compare Math: mean = 70 x = 62 s = 6 English: mean = 80 x = 72 s = 3

Be Careful! Being better is relative to the situation.
What if I wanted to compare race times?

Homework Page 105 (1-15) odd