In a pdf, the definition of adaptive sampling design is written as :

"An adaptive sampling design is one in which the selection of units to include in the sample depends on values of the variable of interest observed during the survey."

What is meant by values of the variable?

I know adaptive sampling design is applicable if the unit is rare.

In an adaptive sampling design, the sampling procedure may be altered during the survey based on the values of the variable of interest that has been observed so far. Assume that the variable of interest is $y$. Also assume that the desired (fixed) sample size is $n_s$ and that you have made $m<n_s$ observations of your variable, $y_1,\ldots,y_m$. Then, in an adaptive sampling design, you would be able to change your selection procedure during the survey depending on the observed $y_1,\ldots,y_m$. So, the "values of the variable" are basically the values of your variable of interest that you have observed until the current time in your survey.
• Isn't adaptive sampling design a sampling procedure ? Isn't this procedure only applicable for rare sampling unit ? And isn't it scheme always to include those neighbors areas to the sampled area with high population density? Then what is meant by the sampling procedure may be altered during the survey based on the values of the variable of interest in your 1st line? I want to mean that the value of variable of interest is always rare in a adaptive sampling and the usual procedure is to select the neighbor areas to the sampled area with high population density. – time Feb 12 '15 at 1:31