Chapter 10 The Art of Data Presentation

Overview 2 Types of Variables Guidelines for Preparing Good Charts Common Mistakes in Preparing Charts Pictorial Games Special Charts for Computer Performance Kiviat Graphs

Types of Variables 3 Type of computer: Super computer, minicomputer, microcomputer Type of Workload: Scientific, engineering, educational Number of processors Response time of system

Guidelines for Preparing Good Charts 4 1) Require minimum effort from the reader Direct labeling vs. legend box 2) Maximize Information Words in place of symbols; cleary label the axes

Guidelines (cont’d) 5 3) Minimize ink No grid lines, more details 4) Use commonly accepted practices origin at (0,0); independent variable (cause) along x axis; the dependent variable (effect) along the y axis; linear scales; increasing scales; equal divisions 5) Avoid ambiguity Show coordinate axes, scale divisions, origin; Identify individual curves and bars

Checklist for Good Graphics 6 Are both coordinate axes shown and labeled? Are the axes labels self-explanatory and concise? Are the scales and divisions shown on both axes? Are the minimum and maximum of the ranges shown on the axes appropriate to present maximum information Is the number of curves reasonably small? Do all graphs use the same scale? Is there no curve that can be removed without reducing information? Are the curves on a line chart individually labeled? Are the cells in a bar chart individually labeled? Are all symbols on the graph accompanied by appropriate textural explanations? If the curves cross, are the line patterns different to avoid confusion? Are the units of measurement indicated? Is the horizontal scale increasing from left to right? Is the vertical scale increasing from bottom to top? Are the grid lines aiding in reading the curves? Does this whole chart add to information available to the reader? Are the scales contiguous? Is the order of bars in a bar chart systematic? If the vertical axis represents a random quantity, are confidence intervals shown? Are there no curves, symbols, or texts on the graph that can be removed without affecting the information? Is there a title for the whole chart? Is the chart title self-explanatory and concise? For bar charts with unequal class interval, is the are and width representative of the frequency and interval? Do the variable plotted on this cart give more information that other alternatives? Does the chart clearly bring out the intended message? Is the figure referenced and discussed in the text of the report?

Common Mistakes in Preparing Charts 7 Presenting too many alternatives on a single chart Max 5 to 7 messages => Max 6 curves in a line charts, no more than 10 bars in a bar chart, max 8 components in a pie chart Presenting many y variables on a single chart

Common Mistakes in Charts (cont’d) 8 Using symbols in place of text Placing extraneous information on the chart E.g., grid lines, granularity of the grid lines Selecting scale ranges improperly Automatic selection by programs may not be appropriate

Common Mistakes in Charts (cont ’ d) 9 Using a line chart in place of column chart line => continuity CPU Type MIPS

Pictorial Games 10 Using non-zero origins to emphasize the difference Mine is much better than yours (emphasize difference) Mine and yours are almost the same (conceal difference)

Pictorial Games (cont ’ d) 11 Using double-whammy graph for dramatization Using related metrics

Pictorial Games (cont ’ d) 12 Plotting random quantities without showing confidence intervals Means of two random variables Means are not enough. Overlapping confidence intervals usually means that the two random quantities are statistically indifferent.

Pictorial Games (cont ’ d) 13 Pictograms scaled by height Wrong scaling: Area(MINE) > 4*Area(YOURS)?? Mine Performance = 2 Yours Performance = 1

Pictorial Games (cont ’ d) 14 Using inappropriate cell size in histograms [0,2)[2,4)[4,6)[6,8)[8,10)[10,12)[0,6)[6,12) Response Time Frequency Normal distributionExponential distribution

Pictorial Games (cont ’ d) 15 Using broken scales in column charts Amplify differences A System Resp. Time BCDE F A System Resp. Time 0 BCDE F

Kiviat graph Eight different metrics are plotted along eight radial lines. The metrics are: 1. CPU busy or CPU utilization HB 2. CPU only busy and none of the I/O channels LB 3. CPU and channel overlap HB 4. Channel only busy, CPU may be waiting for the I/O LB 5. Any channel busy HB 6. CPU wait LB 7. CPU in problem state indicates the time used executing the user’s program HB 8. CPU in supervisor state indicates the time spent in operating the system code. This represents the operating system overhead and is considered bad. LB

17 Kiviat Graphs Radial chart with even number of metrics HB and LB metrics alternate Ideal shape: star CPU Busy CPU in Supervisor State CPU in Problem State CPU Wait Any Channel Busy Channel only Busy CPU/Channel Overlap CPU Only Busy

CPU-bound system has a high CPU utilization with very little I/O usage

I/O-bound system high I/O utilization and low CPU usage

Performance Analysis Rat Holes 20 ConfigurationWorkloadMetricsDetails

Reasons for not Accepting an Analysis 21 This needs more analysis. You need a better understanding of the workload. It improves performance only for long IOs/packets/jobs/files, and most of the IOs/packets/jobs/files are short. It improves performance only for short IOs/packets/jobs/files, but who cares for the performance of short IOs/packets/jobs/files, its the long ones that impact the system. It needs too much memory/CPU/bandwidth and memory/CPU/bandwidth isn't free. It only saves us memory/CPU/bandwidth and memory/CPU/bandwidth is cheap. See Box 10.2 on page 162 of the book for a complete list

Summary 22 Qualitative/quantitative, ordered/unordered, discrete/continuous variables Good charts should require minimum effort from the reader and provide maximum information with minimum ink Use no more than 5-6 curves, select ranges properly, Three-quarter high rule Kiviat Graphs show HB and LB metrics alternatively on a circular graph

Exercise What type of chart (line or bar) would you use to plot: a.CPU usage for 12 months of the year b.CPU usage as a function of time in months c.Number of I/O's to three disk drives: A, B, and C d.Number of I/O's as a function of number of disk drives in a system

Exercise List the problems with the following charts