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# Controlled Experiments (page 3)

By John Wiley & Sons, Inc.
Updated on Jan 1, 2011

### Measuring Results

When the plants have grown to a point that you feel you can judge the results, you end the experiment. Carefully measure the heights of the plants in each planter to compare them, keeping careful records of the measurements for each planter. If a camera is available, take photographs, always with the planters marked in such a way that it is easy to see in the photo which one is experimental and which one is control.

An even better way to measure the plants for comparison is to weigh each one on a laboratory balance scale. Dig up each plant, taking care not to damage the roots. Carefully wash the soil off of the roots and layout each plant on newspaper to dry, keeping the plants for each planter separate while you do the washing and drying. Next, weigh each plant and carefully record the weights for the experimental and for the control.

Now the statistics: For each planter, calculate an average, or mean, of the weights (or measure the lengths if you did not use a balance). You can now compare the averages to determine whether the independent Variable—the baking soda in the water—made a difference in the growth of the plants.

Here a most important problem comes up. We can assume that the mean weights of the plants in each planter will differ somewhat, anyway. So how much difference do you need to show to be able to claim that the baking soda in the water (the independent variable) made a significant difference? Of course, if the plants all died in the experimental planter, you have no problem. Most often, however, the differences are not that obvious. In this case you need to use statistical methods for deciding how much difference is meaningful or significant. The term significant difference should become part of your thinking about any investigation you do.