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| Figure 1. The distribution of heights in
5 cm intervals, of men and women in the Biology 111 class, 2010. N = 106 men, 204 women. |
An Introduction to the Scientific Method
1. Frequency Distributions
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| Figure 1. The distribution of heights in
5 cm intervals, of men and women in the Biology 111 class, 2010. N = 106 men, 204 women. |
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| Figure 2. The mean heights of the Biology 111 students from 2010, ± 95% confidence limit. Data are separated by laboratory section, and amalgamated as one set of data as well (outlined). The error bars (95% CL) are plotted for the whole class, though they are so small they are hidden by the marker. |
The mean (average) of women's heights (N=204) was 166.5 ± 0.9 cm. What this means is that we are 95% sure that the real mean of all women's heights from the whole campus lies within the range of 165.6 cm to 167.4 cm.
2. Correlation
When two parameters are measured, it is not always the case that one parameter relies on the other. Sometimes it is the case that one parameter, for example, rainfall, not only correlates with another parameter, for example, plant growth, but causes it. In the second part of the lab we looked at a case where neither parameter caused the other, but the two were correlated.
The following graph is shown for two reasons. One, to illustrate why we don't always start an axis at zero (lots of wasted space), and two, to illustrate the answer to the frequently-asked question, "Can the y-intercept be negative?" For the line of best fit, yes, the y-intercept can be negative, but in this case, it is biologically meaningless (why?). Normally, one would not plot the line of best fit beyond the minimum and maximum data points, but it's been done here to illustrate the y-intercept.
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| Figure 3. The correlation between growth of the two major axes in the left valves of Atlantic Sea Scallop (Placopecten magellanicus). N=6. The line of best fit has been artificially extended to illustrate the negative y-intercept. |
The real graph of the same data should appear this way:
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| Figure 3. The correlation between growth of the two major axes in the left valves of Atlantic Sea Scallop (Placopecten magellanicus). N=6. |
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Department of Biology St. Francis Xavier University Antigonish, NS Canada B2G 2W5 |