Reading Helsel STATISTICS FOR CENSORED ENVIRONMENTAL DATA USING MINITAB AND R,
at page xviii the author writes:
The Figure i4 shows concentration (y) levels versus distance (x) downstream. What happens when the data are reported using two detection limits of 1 and 3, and one-half the limit is substituted for the censored observations? The result (Figure i5) includes horizontal lines of substituted values, changing the slope and dramatically decreasing the correlation coefficient between the variables. Looking only at these numbers, the data analyst obtains the (wrong) impression that there is no correlation, no increase in concentration.
I don't get how the censored observations are obtained. In particular, in Figure i5, the circled observations (circle is mine) are below detection limit (DL) 1. So, if my understanding is correct, they should get $(1/2)\times 1 = 0.5$ and not $1.5$. What am I missing here?
Even more confusingly, there are observations below 3 that get mapped to (1/2)DL and others that do not....
I am really confused and hope somebody can help me understand what's going on here.
