Locating Sample Plots: Random and Stratified Sampling
Here we describe two ways to locate small sample plots in a larger study area. Refer to the Plot Sampling Protocol for more information.
Random sampling is one way to locate plots. To randomly select plots, scientists first determine how many plots they want, and then use random numbers to locate them. For example, scientists may first walk a random number of steps along the edge of their study area. Then they choose another random number and walk that many steps into the study area. The sample plot is located at the final step.The biggest drawback to random plot selection is the possibility that, by chance, all the plots will be clumped or located near each other. Selecting plots using stratified sampling is a way of avoiding clumping. When using stratified sampling, scientists randomly locate a starting point and then divide their study area into a certain number of equal boxes. They next locate a plot at the corner of each box.
After locating your sampling plots, you will need to build quadrat frames. Then you will count stems of different species on your sampling plots.
- Stakes and flagging
- Locate the corner of your study area.
- Orient yourself so that you will walk along the longest edge of your study area.
- Choose a random number. There are many ways to do this, though one option is to use a stopwatch that measures to the nearest one hundredth of a second. Press the start button, wait a while, and press the stop button. Your two-digit random number is determined by using the digits in the tenth and hundredth places.
- If the edge of your study area is less than 99 paces, you will need a random number between 00 and 99. If the number you choose is larger than the maximum number of steps you can take along the edge of your study area, choose the next random number. For example, if your study area is 60 paces long and you choose random number 71, select another number.
- Beginning at your starting point and continuing along the edge of your study area, walk the number of steps indicated by your random number.
- Turn 90 degrees towards the plot. Choose another random number, and walk the number of steps indicated by this second number. You should be walking into your study area in a direction that is perpendicular to the edge of the plot. Walk in a straight line. Try not to veer to the right or left to avoid shrubs or wet spots.
- The corner of your first sampling plot is located where your foot lands on the last step. You may want to permanently mark the corners of your plot with stakes and flagging. Avoid trampling plants in the plots. Repeat steps 36 to locate additional plots.
- Stakes and flagging
- Map of site
- Surveyors tape (100 m or 50 m) or nonstretch string
- Obtain or make a map of your study site and determine the outside dimensions.
- Decide how many sample plots you will need. Later on, you will divide the study area into a grid of equal-sized squares. You will locate one sample plot in each square.
- Choose a random number and walk that many steps to start the grid. Next divide your study area into a grid of equal-sized squares, one for each sample plot. You may want to first draw out the grid on a map of the site. Then using a survey tape, nonstretch string, or pacing off the correct distances, mark the edges of the squares with stakes and flagging. Avoid trampling and disturbing the study area as much as possible.
- Locate a sample plot at the corner of each of the sections of your study area.
For more information about sampling, read about Invasion Ecology.
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University and Penn State University