I have data for systolic blood pressure over 6 time points for 11 people. Instead of showing the mean response curve, I would like to show individual responses for each person.
A line graph looks too busy, is there another method of showing these individual responses in MS Excel?
You can also add the std dev to the graph. Take a look at Tableau for a range of other options.
1. Using the table of random digits
Grading SRSs chosen from the table of random digits is complicated by the fact that students can find some creative ways to (mis)use the table. Some approaches are not mistakes, but may lead to different students having different “right” answers. Correct answers will vary based on:
• The line in the table on which they begin (you may want to specify one if the text does not).
• Whether they start with, e.g., 00 or 01.
• Whether or not they assign multiple labels to each unit.
• Whether they assign labels across the rows or down the columns (nearly all lists in the text are alphabetized down the columns).
Some approaches can potentially lead to wrong answers. Mistakes to watch out for include:
• They may forget that all labels must be the same length, e.g., assigning labels like 0, 1, 2,..., 9, 10,... rather than 00, 01, 02,....
• In assigning multiple labels, they may not give the same number of labels to all units.
E.g., if there are 30 units, they may try to use up all the two-digit numbers, thus assigning 4 labels to the first ten units and only 3 to the remaining twenty.
2. Using statistical software
The use of computer software or a calculator is a must for all but the most cursory treatment of the material in this text. Be aware of the following considerations:
Standard deviations: Students may easily get confused by software which gives both the so-called “sample standard deviation” (the one used in the text) and the “population
standard deviation” (dividing by n rather than n − 1). Symbolically, the former is usually given as “s” and the latter as “σ” (sigma), but the distinction is not always clear.
For example, many computer spreadsheets have a command such as “STDEV(... )” to compute a standard deviation, but you may need to check the manual to find out which
kind it is.
As a quick check: for the numbers 1, 2, 3, s = 1 while σ . = 0.8165. In general, if two values are given, the larger one is s and the smaller is σ. If only one value is given, and it is the “wrong” one, use the relationship s = σ q n n−1 .
• Quartiles and five-number summaries: Methods of computing quartiles vary between different packages. Some use the approach given in the text (that is, Q1 is the median of all the numbers below the location of the overall median, etc.), while others use a more complicated approach. For the numbers 1, 2, 3, 4, for example, we would have
Q1 = 1.5 and Q3 = 2.5, but Minitab reports these as 1.25 and 2.75, respectively.
Since I used Minitab for most of the analysis in these solutions, this was sometimes a problem. However, I remedied the situation by writing a Minitab macro to compute quartiles the IPS way. (In effect, I was “dumbing down” Minitab, since its method is more sophisticated.) This and other macros are available at my website.
• Boxplots: Some programs which draw boxplots use the convention that the “whiskers”
extend to the lower and upper deciles (the 10th and 90th percentiles) rather than to the minimum and maximum. (DeltaGraph, which I used for most of the graphs in these solutions, is one such program. It took some trickery on my part to convince it to make
them as I wanted them.)
While the decile method is merely different from that given in the text, some methods are (in my opinion) just plain wrong. Some graphing calculators from Sharp draw “box
charts,” which have a center line at the mean (not the median), and a box extending from x − σ to x + σ! I know of no statistics text that uses that method.
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