Lab: Random Sampling

ENVIRONMENTAL SCIENCE
POPULATION ESTIMATE
BY
RANDOM SAMPLING

INTRODUCTION:

When studying a population, it is usually impossible to count every member of the population. One way to estimate the size of a population is by collecting data by random samples. This method works well when estimating a population of plants in a large field. In this lab, you will calculate a population estimate using random sampling, then compare your estimate to the actual population count.

MATERIALS:

Paper
Pencil
Ruler
Scissors
2 Containers

PROCEDURE:

1. Cut 20 slips of paper, each approximately 5 cm x 5 cm.

2. Number 10 of the slips A through J and put them in a container.

3. Label the remaining 10 slips 1 to 10 and put them in a second container.

4. The grid below represents your study area. It is a field measuring 10 m on each side. Each of the grid segments is 1 m x 1 m. Each black dot represents a single sunflower plant.

5. Randomly remove one slip from each container. Record the number-letter combination you drew in the DATA TABLE. Find the grid segment that matches that combination and count the number of sunflower plants in that grid segment. Record the number of sunflowers in the DATA TABLE.
Return each slip to the appropriate container.

6. Repeat step 5 until you have data for 10 different grid segments. These 10 grid segments represent your sample.

DATA TABLE Random sample data

Grid Coordinates

Letter Number Number of
Sunflowers
in grid segment

CALCULATIONS:

7. Find the total number of sunflower plants for the 10-segment sample.

Total number of sunflower plants in the 10 segment sample = ______

Divide the number above by 10 to determine the average number of sunflower plants per square meter in the sample. You didn't have any parts of plants, but for this statistical average, round to the nearest 0.1 plants

Average number of plants per square meter = ______

Multiply the average number of sunflower plants per square meter by 100 (the total number of grid segments in the field) to find the total number of plants in the field. This is your population estimate.

Population estimate = ______

8. Count all the sunflower plants actually shown in the field.

Actual population count = ______

9. Now find the percentage error in your estimate. To calculate percentage error find the difference between the actual count and the estimated count. A difference is always positive, so subtract the smaller of the two numbers from the larger.

Difference = ______

Now divide the difference by the actual count and multiply by 100.

Difference
Actual count

100

% Error

% error = _____ %

ANALYSIS:

10. Why did you draw paper slips, rather than just choosing 10 grid segments?

____________________________________________________________________________

11. What changes could you make to the procedure to reduce your percentage error?

____________________________________________________________________________

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DATA TABLE Random sample data
Grid Coordinates
Letter	Number	Number of Sunflowers in grid segment