Answers to some questions raised by the presentation on the graphical analysis of the interrelationships between waterborne asbestos, digestive system cancer and population density.

Two interesting questions were raised following my presentation (1), and I am pleased to have the opportunity to clarify these issues. The first question dealt with the data in Figure 1. The ques-tioner asked about the interpretation of the axes as they appeared on the graph. In answer to his question it is important to note that all graph coordinate axes pass through the x and y sample means of the displayed data. For example, the mean tract number of the frequency diagram shown in Figure 1 is 216, while the mean number of individuals per square mile is 5270. Thus, the two lines shown in Figure 1 are X 216 and Y 5270 (the symbol a-" is the usual mathematical notation for the phrase "is identical to"). The program that we use, GRAFSTAT, is designed to perform certain scaling and positioning operations automatically. Hence, it applies an initial pass through the data to compute the range of the (X,Y) observations (from which the position of the graph's boundaries are calculated). Simultaneously, it calculates the pair (X,Y) (from which the axes are positioned), and the standard deviation units. For example, the very small number "2" which appears slightly below the mid-point on the Y-axis of Figure 1, is 2 y standard deviation units from the mean point (X,Y). For normally distributed data, as the sample size increases one would of course expect that the number x of all measurements which lie between plus and minus 1.95 standard deviation units of the mean would approach the value x = 5%. The fact that the data depicted in Figure 1 extends more than five standard deviation above, but less than one standard deviation unit below, the point (X,Y) suggests a high degree of skewness for the population density variate. On the other hand, the fairly even spread of data on either side of the mean point in the +-X direction suggests a fairly uniform distribution of the tract number variate. In the second question I was asked how I concluded that the data in Figure 23 appeared to be composed of two population subgroups of the analysis displayed in Figure 25. The questioner thought that they appeared to be an artifact caused by the sparse data in this region and by the smoothing function employed. This is a question about smoothing and bifurca-tion. The method used to smooth the displays shown in …