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
- 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
- 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.
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.
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
- Locate a sample plot at the corner of each of the sections of
your study area.
For more information about sampling, read about Invasion