PSY 201 1 Introduction to Statistics in Psychology PSY 201 Greg Francis PhD Department of Psychological Sciences Psychological Sciences Building Room 3174 765 494 6934 email gfrancis purdue edu http www1 psych purdue edu gfrancis Classes PSY201 Homework 2 Due 01 September 2010 Hand in your answers on a separate sheet of paper 1 A class of students recorded the number of years their families had lived in their town Here are two graphs that students drew to summarize the data Which graph gives a more accurate representation of the data Why 2 Using the following distribution of scores find the percentile rank corresponding to X 7 0 X f 10 2 9 8 8 4 7 6 6 4 5 1 3 Using the following distribution of scores find the 50th percentile X f 20 24 2 15 19 3 10 14 3 5 9 10 0 4 2 PSY 201 2 4 For the following set of N 20 scores construct a frequency distribution table using an interval width of 5 points 14 8 27 16 10 22 9 13 16 12 10 9 15 17 6 14 11 18 14 11 5 Find each value requested for the distribution of scores in the following table a n b P Xi c P Xi2 X 5 4 3 2 1 f 1 2 4 3 1 6 The following frequency distribution presents a set of exam scores for a class of N 20 students X f cf c 90 99 4 20 100 80 89 7 16 80 70 79 4 9 45 60 69 3 5 25 50 59 2 2 10 a Find the 30th percentile b Find the 88th percentile c What is the percentile rank for X 77 d What is the percentile rank for X 90 7 Mental imagery has been shown to be very effective for improving memory In one demonstration study two groups of participants were presented with a list of 20 words One group was given instructions to use mental images to help them remember the words and the second group was given no special instructions After the list was presented each person was asked to recall as many words as possible The scores for the two groups are as follows Imagery group 18 19 18 17 19 13 20 17 18 18 20 18 14 19 16 No instruction group 11 12 16 12 10 13 14 12 10 8 13 10 11 14 12 a Sketch a histogram showing the distribution of scores for each group two separate graphs b Describe each group by identifying the shape of the distribution and the approximate center of the distribution c Based on the appearance of the two distributions what is the effect of using mental images on the level of performance for this memory task PSY 201 3 8 For the following set of scores 2 3 2 4 5 2 4 2 1 7 1 3 3 2 4 3 2 1 3 2 a Construct a frequency distribution table b Sketch a polygon showing the distribution c Describe the distribution using the following characteristics i What is the shape of the distribution ii What score best identifies the center average for the distribution iii Are the scores clustered together or are they spread out across the scale 9 A survey given to a sample of 200 college students contained questions about the following variables For each variable identify the kind of graph that should be used to display the distribution of scores histogram polygon or bar graph Justify your choice with one or two sentences a number of pizzas consumed during the previous week b size of T shirt worn S M L XL c gender male female d grade point average for the previous semester e college class freshman sophomore junior senior 10 Using requested X 95 99 90 94 85 89 80 84 75 79 70 74 65 69 60 64 50 59 the following frequency distribution find each of the percentiles and percentile ranks f 2 5 12 15 10 2 2 1 1 cf 50 48 43 31 16 6 4 2 1 c 100 96 86 62 32 12 8 4 2 a Find the 30th percentile b Find the 80th percentile c Find the percentile rank of X 60 d Find the percentile rank of for X 78