MIT Guide to Science and Engineering Communication 2ed - J Paradis (MIT 2002) Episode 4 pps

These forms, whether developed from a database and computer graphics package such as Excel (Microsoft Office), SAS (hwww.sas.comi), or Matlab (hwww.mathworks.comi) or even roughed out on a pad of grid paper help you examine your data for important trends. They are think- ing aids. Depending on the point you’re making, you could create a va- riety of graphs out of the data in Figure 6.3. Two of many possibilities are shown in Figure 6.4a, b. The simple line graph compares two trends in the data; the semilogarithmic chart compares rates of change for two series of values that differ greatly in magnitude. Choosing the Type of Graphic Modern data graphics libraries offer an immense varie ty of options for analysis and presentation; familiarize yourself with those that are used in Figure 6.3 Tables are the simplest visual format and preserve the original data. Each cell represents a full sentence. Tables do not, however, convey visual patterns and may obscure significant events or trends. Developing Graphics 65 Figure 6.4 Graphics can manipulate data. Many different graphics may be prepared from the tabular data in Figure 6.3, depending on the writer’s purpose and audience. The line chart (a) focuses on the comparative flux of one element at different ocean depths. The semilogarithmic chart (b) plots the mean values of two data series of very different magnitudes. (a) (b) 66 Developing Graphics your field. A simple, widely available package like Excel enables you to visualize data in a variety of line graphs, bar charts, surface charts, scat- tergrams, logarithmic charts, pie charts, and others. The more elaborate suites of programs in Matlab and SAS apply to a variety of specialized fields in the sciences, applied sciences, and social sciences. Choose a graphic design that supports your argument and works effectively with your data. The choices range from simple line graphs that track a set of values for a given item to more complex series that plot correlations of two interrelated variables. Some of the more common kinds of graphic visualization include: . Items with Different Values. Tracing the different values for one or more items is one of the most common kinds of graphic display (Figure 6.5). These values may be plotted as bars on a bar or column chart or data points on a line graph. An example might be a series of values for costs of a specific service such as electrical power in different parts of the country. Bar charts are the most straightforward way to visualize this kind of discontinuous data. Sometimes called column charts if they are aligned verti cally, bar charts are effective for showing a series of discrete values. Pictographs, which are bar charts whose bars are made up of symbols, are often effective with general audiences. You might use a line graph, however, if you are showing a trend in the changing values for an item: the changing fructose content in a species of plant, for example, as it is sampled at latitudes progressively closer to the equator. . Time Series. Time series, probably the most widely used of all data graphs, plot a changing value for one or more items in relation to some unit of tim e (see Figure 6.4). This kind of continuous data, marked by uninterrupted extension in time, is best represented in a line graph that connects the data points. Data appropriate for time series might be the size (area) of a bacterial culture per one-hour period under a defined set of conditions. Other, less continuous kinds of time-related data, such as the sales volume for an item in liters, kilograms, or dollars over several months, may be represented better in a bar chart. It is possible, of course, to connect the ends of the bars (at their midpoints) in a bar chart with a line to emphasize a trend in discontinu ous data. . Percentages. Line graphs, bar charts, and pie charts can all express percentages (Figure 6.6). Percentages may be parts of a whole, or they may be percentiles, which express more subtle distinctions. For example, you could represent the pe rcentage of oz one in air samples at 100-meter increments between 5,000 and 10,000 meters, or you could graph the percentage of a specific population or percentile of students scoring A, B, Developing Graphics 67 Figure 6.5 Typical graphs of items with different values. 68 Developing Graphics Figure 6.6 Typical graphs of percentages. C, D, and E points on a given exam. If the percentage readings are taken at very frequent intervals, then a line graph is a good choice. For larger intervals, the bar chart is more appropriate. For a simple percentage breakdown of a given sample, the pie chart or the 100 percent bar chart are the most effective. . Comparison. In plotting comparisons (Figure 6.7), your challenge is to design a graph that displays a series of trends simply and clearly. Pie charts are not as effective for comparing values as the line graph or bar chart. Multiple data fields are very effective when the scales of the data trends are changing or when there are too many trends to fit into a single data field. . Correlation. Correlations demonstrate or suggest the mutual influ- ence (covariance) of two variables (Figure 6.8a). The variables A and B are normally two different items or two aspects of a single item. The correlation in Figure 6.1, for example, relates the strength of a weld to the motion of the welded parts during welding. Electrical output is a function of resistance, and bacterial growth is a function of ambient temperature. Correlation represents these linkag es. Each variable depends on the other, or both variables depend on yet another phenomenon. The covariance may be positive or negative, where an increase in A shows a proportional increase or decrease in B. The in- dependent variable is plotted on the horizontal scale for a line graph or scatter chart. The line graph and scatter chart are most effective for dis- playing correlation because each data point is an expression of the two variables. Be careful not to confuse correlation with comparison. Correlated variables are supposed to be interdependent, although proof of their in- terdependence is often the key issue in the graphic display. . Ratios and Rates of Change. The semilogarithmic scale (Figure 6.8b) is an effective way of plotting data for an item whose values vary greatly. For example, if the series of data points jumps from the 10 1 range to the 10 3 range, the trend cannot be seen clearly on a standard scale because a scale designed for the 10 3 range would not show variations at the 10 1 magnitude. With the semilogarithmic scale, which shows rate of change, different magnitudes may be plotted in detail and with clarity. An increasing slope shows increasing rate of change in relation to prior values. . Frequency Distributions. Distribution graphics illustrate the spread of a population, as in the bell curve distribution demonstrating proba- bility (Figure 6.8c). In construct ing frequency graphs, you select an in- cremental unit (the independent variable) that will best reveal the shape 70 Developing Graphics Figure 6.7 Some typical ways of graphing comparisons. Figure 6.8 Some common varieties of analytical graphs: (a) correlation of two variables, (b) rate of change, (c) frequency, and (d) net differences. 72 Developing Graphics Figure 6.8 (continued) Developing Graphics 73 Figure 6.8 (continued) 74 Developing Graphics [...]... Table 1-1 , 1-2 , 1-3 or 1.1, 1.2, 1.3, and so on) Align the values in the vertical columns along the decimal If you are missing a value for a cell, Developing Graphics 77 leave the space blank Be sure to identify and discuss the table in your main text Graphs Designing and preparing graphics takes time, even with the powerful assistance of spreadsheets The table and charts shown in Figures 6.3 and 6 .4 will... modifiers next to words they modify The modifier ‘‘0.25 uF’’ applies to capacitors and should be better placed 4 Data points Be sure that data points are large enough not to be obscured when reduced Squares, circles, and triangles are more effective 84 Developing Graphics Developing Graphics 85 Figure 6.11 Make the graphic support the argument In (a) the author attempts to use four pie charts to compare... from development to marketing, you’ve been made the publication manager, and you’ve just hired two new staff writers Your first task is to instruct them about specifications for the company’s documentation, most of which are stored only in your memory You need quickly to develop document standards so that all work from your department is consistent To complicate matters, you want documents to be available... Figure 1, 2, 3 (for larger documents with chapters, number figures by chapter as Figure 1-1 , 1-2 , 1-3 or Figure 1.1, 1.2, 1.3, and so on) If you are using data from a copyrighted source, get written permission to use it and give credit to your source at the end of the caption You should also label the variables on the x and y scales of the graph along with their units of measurement If your graph includes... clear to your target audience Here are some final things to check before you submit for publication Are Your Curves and Scales Accurate? The accuracy of data is often an issue Authors face two special concerns: the technical accuracy of the data and the presentational accuracy of the visual Check over your data to be sure you have derived it effectively and review your visualization of the data to be... design drawings, function plots, and the like The drawing shown in figure 6.10 is a typical no-frills illustration of an apparatus addressed to a technical audience By contrast, administrators often prefer much-simplified graphics that address their managerial concerns for understanding a situation and allocating resources Simple tables, line graphs, bar charts, pie charts, and diagrams, produced by spreadsheet... equal to a given task You may be displaying the right information without effectively making the point Be prepared to discard or redesign a graphic if it is not doing the job We see an example of this problem in Figure 6.11a, which attempts to use pie charts comparatively The author is trying to argue that aluminum will continue to hold its own with composites (CFCs) and Titanium as a material used to. .. ‘‘List of Figures and Tables’’ in a floppy file that gives the full number, title, and caption for each of your visuals (List tables and figures separately.) If you are producing your own documents, titles appear below figures and above tables 2 Symbols Check to make sure that you are using the standard units and symbols of your discipline The symbol for ‘‘Hertz’’ is ‘‘Hz’’ (not ‘‘HZ’’) 3 Title and caption wording... user of Excel a couple of hours to prepare Crude draft graphics should suffice until you’re ready to commit the time needed to complete a finished version Preparing a graph involves a series of steps: 1 Select a design that demonstrates the point you want to make Early in the project, read over your notes, outlines, or drafts and think carefully about which key ideas you want to support with your graphics... graphics should be used to support an argument or illustrate a mechanism or process Study other documents in your subject area to get ideas for effective approaches to displaying data Using your database, you may want to experiment with various graphic templates to see which ones work well with the data Keep in mind that developing finished graphics is time consuming, space is limited, and reproducing graphics . trend you want to show. Plot the data. . Line graphs. Place the independent variable on the horizontal x-axis and the dependent variable on the vertical y-axis. The intersection of the x- and y-axes is. by chapter as Figure 1-1 , 1-2 , 1-3 or Figure 1.1, 1.2, 1.3, and so on). If you are using data from a copyrighted source, get written permission to use it and give credit to your source at the. in Matlab and SAS apply to a variety of specialized fields in the sciences, applied sciences, and social sciences. Choose a graphic design that supports your argument and works effectively with