Random Sampling

What You Need | The Steps To Follow

What You Need

• Your plexiglass discs or compact discs.
  
  
• A petri dish to hold the disc.
  It is a very good idea to hold the disc in place when you are identifying the number of different organisms present. Our method was to glue four pieces of tubing to the petri dish. The height of each piece is approximately the height of the side of the petri dish.
  
  
• A stereomicroscope.
  
  
• The sampling grid printed on transparency film (please note the number of grids).
  
  
• A random number table.
  You can find this in almost any basic statistics book. There are even computer programs that will generate a series of random numbers.
  
  

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The Steps To Follow

1. Place the sampling grid in the petri dish.
   
   
2. Place your disc in the petri dish and cover with water from the sampling site (note: for very thick biofilms, you may need to reverse these two steps and place the grid on top of the disc).
   
   
3. Use your random number table to select a grid to sample. To choose your starting point, close your eyes and touch the table with a finger.
   
   
4. Going left to right, determine the next 3 digit number.
   
   
5. Determine if it is between 1 and 100, inclusive. In other words, is it one of the numbered sites on our sampling grid.
   
   
6. If the number is within this range, locate this grid using the steromicroscope and record the number of different organisms attached to the disc (i.e., part of the biofilm).
   
   
7. If the number is outside this range, go to step 3.
   
   
8. Repeat steps 3-5 three times (i.e., take 3 random samples).

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