CALCULATION OF DATA 191 



A direct comparison of two experiments is possible only if the same 

 amount of living material was used in both cases. An indirect comparison 

 can be made by recalculating both sets of results in terms of some stand- 

 ard amount of material. Simply weighing the cells in both experiments 

 makes such a comparison possible since the results can be expressed in 

 terms of amount of change per gram of cells. Cells or tissues weighed in 

 the living condition yield a "fresh weight." 



Suppose we were measuring the amount of a certain ion absorbed by 

 slices of potato tuber, and we wished to compare two batches of potatoes. 

 The absorption of ions could be expressed as a number of grams (or 

 milligrams) per gram (fresh weight) of tissue. Because one batch of 

 potatoes might contain a much larger amount of water, relatively, than 

 the other, a direct comparison on the basis of fresh weight could be 

 misleading. A more realistic measure of the amount of potato tissue, in 

 this case, is the dry weight obtained by drying the tissue in an oven 

 after the experiment is completed. Alternatively, one sample of each 

 batch of potatoes could be used in the experiment and another sample 

 of each, equal in fresh weight and volume, could be dried. Even this 

 method is not entirely adequate. The absorption of ions sometimes de- 

 pends upon metabolic activity, which in turn depends upon the relative 

 concentration of enzymes present. If one kind of potatoes has a large 

 dry weight, but most of this weight is metabolically-inactive starch, the 

 results expressed on the dry weight basis are not very useful. A better 

 comparison would be based on the amount of protein nitrogen per unit 

 of potato tissue. The amount of protein would indicate the amount of 

 enzymes present; thus, results expressed as milligrams of ions absorbed 

 per unit of nitrogen are more realistic. This example illustrates a rather 

 common dilemma in biological experiments. As the experimental results 

 become more directly comparable, it becomes necessary to make two sets 

 of measurements in each experiment: measurements of the amount of 

 living material, as well as the measurement of the phenomenon being 

 studied. 



Manipulations of raw data 



The purpose of an experiment is to answer a hypothetical question, 

 but the results are just a set of numbers. The question cannot be an- 

 swered unless the numbers are in a form directly related to the form in 

 which the question was stated. Several kinds of manipulations of the raw 



