A pool refers to a group of individuals subject to a specific set of random selection parameters, such as the rate (i.e. 50%) and periodicity (monthly) of selections.

The rate is the number of selections as a percentage of the pool size. For example, if there are 100 people in the pool, and the annual rate is 50%, then 50 selections will occur over a years time. Because the process is random, it is probable that a significant number of the 50 selections will “repeat”, meaning that some people get picked more than once. So a random rate of 50% of a 100 person pool means that you’ll conduct 50 drug tests, not test 50 different people.

The program period refers to the period of time during which the random rate will be calculated.The easiest program period to use is one year, however, it’s possible and sometimes advisable to have shorter program periods. The program period is divided into a specific number of selection periods, which is called the frequency. Testing activity may fluctuate over the course of the program period, but by the time the period closes, the number of completed tests should equal the random rate.

Frequency is the number and spacing of selection periods during the program period.  Typical frequencies are month, weekly, quarterly, or daily.  Other frequencies are possible and sometimes helpful.  A high frequency of selections, i.e. daily, results in a very high level of deterrence. However, it also tends to more difficult to administer.  As a general rule for establishing deterrence, you should use the highest frequency possible given your administrative capabilities.  For example, if all of your pool members are located at a single facility with on-site collection capabilities, then weekly, or even daily selections are possible.  But if the same number of people are spread out over a large geographic area with diverse work schedules, monthly or quarterly selections may be more appropriate. 

The key factor that helps determine frequency is the ability to locate, notify, and collect a sample from the individual selected for testing.  That ability is driven by your communication abilities, management practices, geographical structure, and collector arrangements.  Of course, all of these are related to the cost of the testing program.

The selection period is an interval within the program period for which a given number of random selections are performed and their corresponding tests completed.  Typical selection periods are one month, one week, one quarter, or one day.  For example, if you have chosen a frequency of “monthly” each month would be a new selection period.  There are a couple of important things to remember about selection periods:

1)      When using simple random sampling with replacement, the prior selection periods have absolutely no impact on the current selection period!  (Hint:  Think of it as a new roll of the dice- the dice have no memory of previous roll.)

2)      The easiest and most objective way to administer testing is to excuse all pending tests at the end of each selection period.  “Carrying over” can introduce all sorts of problems, most of which result from bias. Remember, random testing is about performing a specific number of tests on the subject population.  If you’re finding it difficult to accomplish the desired number of tests in each selection period, you may have to adjust management practices, communication, or the logistics of collection.  You may also simply need to increase the number of selections per period (over sampling), or, at the start of the next program period, change the frequency.

Over sampling refers to the practice of selecting more people for testing than the rate requires. This is done in anticipation of some number of tests not being completed.  Over sampling is required in almost every random testing program because it is simply not possible to conduct a test on every person that is picked by the computer.  People get sick, go on vacation or leave, change responsibilities, or are otherwise unavailable for testing.  An over sampling rate of 20% is quite common.  It can be much higher or lower, depending on the situation.  Again, the emphasis of random testing is completing a given number of tests during the program period in an unbiased fashion. Some measure of over sampling is required to meet that goal.

Random selection is a mathematical process driven by several parameters: pool membership, the program period, rate, and frequency.

STEP 1. A “pool group” is created.  The pool group includes those personnel subject to random testing.  The operator also specifies the rate of random testing, or a specific number of pool members to be selected each period.  For example, the Department of Transportation requires a 50% annual testing rate.  This means that, over the course of one year, at least 50 drug tests must be conducted for every 100 employees in the pool.

It is important to understand that the 50 tests do not have to be conducted on 50 different individuals. In fact, this is highly improbable, if not impossible.  At a 50% selection rate, the actual probability is that 37 or 38 different individuals will be selected for the 50 tests.  This means that 12 or 13 of the 100 individuals in the pool will be selected at least twice or more.

STEP 2. Before beginning the selection process, Drug Test America figures out how many tests need to be conducted for the “selection period.”  The selection period is usually a week or month.  The Department of Transportation requires that each member of the pool have an equal chance at being selected for a test every selection period.  When figuring out how many tests are needed for the period, Drug Test America takes into account absenteeism, incomplete tests, etc. to make sure that the minimum number of required tests is accomplished.

STEP 3. Drug Test America uses a random algorithm, or mathematical equation, to assign an “index number” to every one in the pool.  The employee’s index number is usually different every selection period, however, it is possible for the computer to assign the same index number two or more periods in a row.  The number of index numbers is always equal to the number of people in the pool for the selection period.  The index number becomes the “identity” of each member of the pool group for the selection period.

For example, if the pool group has 100 members, then each member in the pool will receive a randomly assigned index number between 1 and 100.

STEP 4. Using a random algorithm, Drug Test America generates a series of random numbers equal to the number of tests required for the period. Drug Test America then looks at the index numbers that are randomly assigned to the pool group members and matches up the numbers.

For example, if Drug Test America determined that 5 tests were needed for the period in a pool group of 100 members, it would pick at random 5 numbers between  1 and 100.  For illustrative purposes, let’s assume that the numbers 34, 45, 67, 35 and 10 were picked by Drug Test America.  Drug Test America would then search through the 100 index numbers and find out which pool group members were assigned the index numbers of 34, 45, 67, 35 and 10. Those five individuals would be selected for a test.

IMPORTANT:  THIS PROCESS CANNOT BE UNDONE.  ONCE Drug Test America HAS ASSIGNED INDEX NUMBERS AND MADE SELECTIONS, A PERMANENT RECORD FOR EACH SELECTION IS CREATED.

It is also important to know that the random algorithm used by Drug Test America has been thoroughly tested and documented.  Drug Test America’s random number generator verification is available upon request. Statistical analysis has also determined that computer algorithms are the “best” random generators because they are free from physical biases and can thoroughly document the random selection process. 

If the explanation above seems a little confusing, the following example will help illustrate how Drug Test America selects individuals for random tests:

Let’s assume that there are 52 people in a room that are subject to random testing.  Let’s also assume that 5 people need to be picked for random tests. We can accomplish this goal fairly with two decks of playing cards.  First, we would shuffle both decks of cards.  We then take the cards from one deck and pass out one card, face down, to each person in the room.  Next, we would draw five cards from the second shuffled deck and place them face up on a table.  Everyone in the room would then turn their playing card face up.  The five cards on the table from the second deck will match up to five individuals in the room holding cards from the first deck. These five individuals are now “picked” for a test.

We could repeat this exercise time and time again, shuffling both decks each time and passing out the cards. The odds are that some individuals will never get “picked”, and, in like manner, some individuals will be picked several times.

Why are some people picked for testing more than once and others are not picked at all?

The card analogy explains it in detail, but there are many other familiar random processes that help you understand how this happens.  When you flip a coin thousands of times, it’s probable that you’ll get “heads” as many times as “tails”.  Each incidence of flipping the coin has nothing to do with the previous flip. However, it is unlikely that each flip will have the opposite result of the preceding flip.  You may get a whole series of “heads” before another “tails” flip. So, if everybody in your pool participated long enough, it’s likely that selections would end up being evenly distributed.  However, in the real world, people move in and out of the pool at a rate that makes that impossible. This is especially true given the relatively low rates of selection used in drug testing (i.e. 10%-50%).

Some words of caution: It’s possible for someone to have increased chances of being picked if they are entered in the pool more than once.  The computer won’t let someone with the same unique ID be entered in the pool twice, but if a mistake is made whereby an individual exists in the pool under two or more unique ID’s, the odds of that person being picked go up (like having two raffle tickets).

Another common problem is not removing “unavailable” people from the pool on a regular basis.  If the pool contains ID’s of individuals that cannot be tested (no longer working, extended leave, etc.), those that are available will be subjected to a higher incidence of testing events.