Field sampling lab

RANDOM SAMPLING EXERCISE

Scientists cannot possibly count every organism in a population. One way to estimate the size of a population is to collect data by taking random samples. In this activity, you will look at how data obtained from random sampling compare with data obtained by an actual count.

Procedure

1.       Tear a sheet of paper into twenty slips, each approximately 4 cm x 4 cm.

2.       Number ten of the slips from 1 through 10 and put them in a small container. 

3.       Label the remaining ten slips from A through J and put them in a second container. 

The grid shown below represents a meadow hypothetically measuring 10 m on each side. Each grid segment is 1 meter x 1 meter.  Each circle represents one sunflower plant.

4.       Randomly remove one slip from each container. Write down the number-letter combination and find the grid segment that matches the combination. Count the number of sunflower plants in that grid segment. Record this number on the data table. Return each slip to its appropriate container. 

 

 

5.       Repeat Step 4 until you have data for 10 different grid segments (and the table is filled out). These 10 grid segments represent a sample. Gathering data from a randomly selected sample of a larger area is called sampling. 

6.       Find the total number of sunflower plants for the ten-segment sample. To do so, add all the grid segment sunflowers together and divide by ten to get an AVERAGE number of sunflower plants per grid segment. Record this number in the table. Record this number in a data table similar to the one below that can be easily added to your blog page or use this table, scan, and add.

7.       Multiply the average number of sunflower plants by 100 (the total number of grid segments) to find the total number of plants in the meadow based on your sample. Record this number in a data table similar to the one below that can be easily added to your blog page or use this table, scan, and add.

Random Sampling Data			Actual Data
Grid Segment (number and letter)	Number of Sunflowers		Total number of sunflowers ___26___ (count by hand)   Average number of sunflowers (divide total by 10) Per grid __2.6___
J-1	1
I-10	3
F-8	2
C-4	3
H-9	2
E-10	3
C-10	3
B-3	3
H-10	3
A-7	3
Total Number of Sunflowers	26
Average (divide total by 100)	.26
Total number of plants in meadow (multiply average by 100)	228

8.        Now count all the sunflower plants actually shown in the meadow. Record this number in the data table. Divide this figure by 100 to calculate the average number of sunflower plants per each grid. 

Analysis

1.       Compare the total number you got for sunflowers from the SAMPLING to the ACTUAL count.  How close are they? 

There was a total of 26 sunflowers that I sampled. Which means that there was an average of 2.6 sunflowers per grid. Upon counting the actual amount of sun flowers, it was pretty accurate. The actual count was 2.28, which means it’s only off by .32.

 

2.       Why was the paper-slip method used to select the grid segments?

When doing a lab in the field it’s easier to choose sections of the measured grid rather than counting each one indivudally. You won’t be 100% accurate but you can get a pretty good idea

3.       A lazy ecologist collects data from the same field, but he stops just on the side of the road and just counts the ten segments near the road. These ten segments are located at J, 1-10. When she submits her report, how many sunflowers will she estimate are in the field?

She will only estimate that there are 70 sunflowers.

 

4.       Suggest a reason why her estimation differs from your estimation.

Because she didn’t venture into the field. If she had stopped for 30 minutes and just walked the sun flower field, choosing 10 segments, she would have a more accurate sampling.

 

5.       Population sampling is usually more effective when the population has an even dispersion pattern. Clumped dispersion patterns are the least effective.  Explain why this would be the case.

Because when the dispersion pattern is even you can get a better “head” count. If it’s clumped together, you risk the possibility of counting the same spot more than once.

 

6.       Describe how you would use sampling to determine the population of dandelions in your yard.

I’d make a grid of 5 x 5 with yarn. Then sectioning the 5 x 5 into 25 equal blocks. After which I would count the amount of dandelions in each square. After I’ve counted 5 different squares I would divide it by 5. Then I’d multiply the average by 25 to get my total.

 

7.       In an area that measures five miles by five miles, a sample was taken to count the number of desert willow trees. The number of trees counted in the grid is shown below. The grids where the survey was taken were chosen randomly. Determine how desert willow trees are in this forest using the random sampling technique. Show your calculations.

	7
				3
			5
11		9

 

There are 35 willow trees that were recorded. Divide that number by 5 as that’s how many samples there are. 7 is the average number. Multiply 7 by how many blocks in the segment. 25. And the total number willow trees is approx.. 175

 

(11+7+9+5+3)=35/5=7*25=175

 

 

Reference

 

Biology Corner. Random Sampling. 2014 Apr 6. Web.