Basic Statistics

1. Basic Statistics Measures of Central Tendency

2. Characteristics of Distributions • Location or Center • Can be indexed by using a measure of central tendency • Variability or Spread • Can be indexed by using a measure of variability

3. Consider the following distribution of scores: How do the red and blue distributions differ? How do the red and green distributions differ?

4. Consider the following distributions: How do the green and blue distributions differ?

5. Consider the following two distributions: How do the green and red distributions differ?

6. Characteristics of Distributions • Location or Central Tendency • Variability • Symmetry • Kurtosis

7. Measures of Central Tendency Summarizing Data The Mean The Median The Mode Give you one score or measure that represents, or is typical of, an entire group of scores

8. Most scores tend to center toward a point in the distribution. frequency score Central Tendency

9. Measures of Central Tendency Are statistics that describe typical, average, or representative scores. The most common measures of central tendency (mean,median, and mode) are quitedifferent in conception and calculation. These three statistics reflect different notions of the “center” of a distribution.

10. “The Mode” The score that occurs most frequently In case of ungrouped frequency distribution

11. Unimodal Distribution -One Mode- Bimodal Distribution –Two Modes-

12. Mode and Measurement Scales Can you find a mode for each data? Nominal Scale Ordinal Scale Interval Scale Ratio Scale 3 1 2 1 3 3 2 3 3 3 1 2 1 2 3 3 2 1 2 3 2 4 1 2 3 4 4 3 4 3 2 4 4 2 1 2 4 4 3 2 3 4 112 56 68 56 39 56 44 56 45 56 75 81 67 59 112 132 112 113 112 150 125 114 Nationality 1=American 2=Asian 3=Mexican Football Poll 1=first 2=second 3=third 4=fourth IQ score Weight

13. “The Mode” • It is not affected by extremely large or small values and is therefore a valuable measure of central tendency when such values occur. • It can be found for ratio-level, interval-level, ordinal-level and nominal-level data

14. “The Median” The Median is the 50th percentileof a distribution - The point where half of the observations fall below and half of the observations fall above In any distribution there will always be an equal number of cases above and below the Median. Oh my !! Where is the median? Location

15. For an odd number of untied scores (11, 13, 18, 19, 20) 11 12 13 14 15 16 17 18 19 20 The Median is the middle score when scores are arranged in rank order Median Location = (N+1)/2 = 3rd Median Score = 18

16. For an even number of untied scores (11, 15, 19, 20) 11 12 13 14 15 16 17 18 19 20 The Median is halfway between the two central values when scores are arranged in rank order Median Location = (N+1)/2 = 2.5thScore Median = (15+19)/2 = 17

17. The Median of group of scores is that point on the number line such that sum of the distances of all scores to that point is smaller than the sum of the distances to any other point. • There is a unique median for each data set. • It is not affected by extremely large or small values and is therefore a valuable measure of central tendency when such values occur.

18. The Median • Can be computed for: • Ordinal-level data • Interval-level data • Ratio-level data

19. Median and Levels of Measurement 1 2 1 3 3 2 3 3 3 1 2 1 2 3 3 2 1 2 3 2 1 2 3 4 4 3 4 3 2 4 4 2 1 2 4 4 3 2 3 4 112 132 112 113 112 150 125 114 68 56 39 56 44 56 45 56 75 81 67 59 No Yes Yes Yes Nationality Football Poll IQ score Weight Can you find a median for each type of data?

20. The Mean

21. The Population Mean For ungrouped data, the population mean is the sum of all the population values divided by the total number of population values. To compute the population mean, use the following formula. Sigma Individual value Population mean Population size

22. The Sample Mean For ungrouped data, the sample mean is the sum of all the sample values divided by the number of sample values. To compute the sample mean, use the following formula. Sigma (Summation) Sample Mean Individual value Sample size

23. Characteristics of The Mean Center of Gravity of a Distribution

24. Center of Gravity of a Distribution 1 2 3 4 5 6 7 8 Mean

25. How much error do you expect for each case? Deviation Scores -6 25 -4 27 31 -2 0 31 29 2 33 The Mean 6 4 35 37 Data set

26. On average, I feel fine It’s too hot! It’s too cold!

27. The Mean of group of scores is the point on the number line such that sum of the squared differences between the scores and the mean is smaller than the sum of the squared difference to any other point. If you summed the differences without squaring them, the result would be zero.

28. Mean and Measurement Scales Every set of interval-levelandratio-leveldata has a mean. Can you find the Mean for the following data sets? Nominal data Ordinal data Interval data Ratio data 1 2 3 1 2 3 1 2 3 1 2 3 2 2 YES NO YES NO Nationality 1=American 2=Asian 3=Mexican Football Poll 1=first 2=second 3=third IQ Test Weight

29. All the values are included in computing the mean.

30. A set of data has a unique mean and the mean is affected by unusually large or small data values [outliers]. 1 1 3 3 5 7 9 9 5 5 6 5 4 5 5.5 The Mean

31. Every set of interval-level and ratio-level data has a mean. • All the values are included in computing the mean. • A set of data has a unique mean. • The mean is affected by unusually large or small data values. • The arithmetic mean is the only measure of central tendency where the sum of the deviations of each value from the mean is zero.

32. The Relationships between Measures of Central Tendency and Shape of a Distribution

33. Normal Distribution Symmetric Unimodal Mean = Median = Mode

34. Positively Skewed Distribution Mode Median Mean Mode < Median < Mean The median falls closer to the mean than to the mode

35. Negatively Skewed Distribution Mode Median Mean Mode > Median > Mean The median falls closer to the mean than to the mode

36. Bimodal Distribution Mode1 Mode2 Mean = Median Mode1 < Mean = Median < Mode2

37. SUMMARY There are three common measures of central tendency. The mean is the most widely used and the most precise for inferential purposes and is the foundation for statistical concepts that will be introduced in subsequent class. The mean is the ratio of the sum of the observations to the number of observations. The value of the men is influenced by the value of every score in a distribution. Consequently, in skewed distributions it is drawn toward the elongated tail more than is the median or mode. The median is the 50th percentile of a distribution. It is the point in a distribution from which the sum of the absolute differences of all scores are at a minimum. In perfectly symmetrical distributions the median and mean have same value. When the mean and median differ greatly, the median is usually the most meaningful measure of central tendency for descriptive purposes. The mode, unlike the mean and median, has descriptive meaning even with nominal scales of measurement. The mode is the most frequently occurring observation. When the median or mean is applicable, the mode is the least useful measure of central tendency. In symmetrical unimodal distribution the mode, median, and mean have the same value.