We discovered in the last post that samples may be obtained in many ways.

The goal is to obtain a representative sample, though (as far as possible).

Based on the manner in which samples are obtained, statisticians may broadly classify sampling techniques as either “**Probabilistic**” or “**Non-Probabilistic**“.

What does “**Probabilistic**” mean? In the context of sampling, it simply **means that each subject (or sampling unit) has a known probability (or likelihood) of selection. The probability (or likelihood) of selection must not be zero. **

Please note that knowing the likelihood (or probability) of selection does not mean we know who will be selected: If I know that the probability of selection is 0.5 (or 50%), I know that half of the population will be selected, but I do not know who will be in that 50%.

**“Probabilistic” sampling techniques (often) involve the term “Random”.**

What is meant by “Random”? In common understanding, random means arbitrary or ad-hoc. In statistics, however,** “Random” means unpredictable or unguessable**. (This automatically gives eligible members of a population similar chance of being selected.)

**Examples**:

Let’s assume that I need to choose 10 individuals from a crowd of 100 people. If I lined up everyone and then chose every 10th person, that is *not* **“Random”**, because anyone could guess who’d be selected next.

If, on the other hand, I assigned each person a number from 1 to 100; made 100 chits numbered from 1 to 100; put all the chits in a bin; shook the bin vigorously; and picked any 10 chits out of the bin, that **is** **“Random”**.

**“Non-Probabilistic” Sampling** techniques: Those in which **one or both of the following may be true**

**i. The probability (or likelihood) of selection is not known**

**ii. The probability (or likelihood) of selection is zero for some members of the population.**

How does one decide whether to use probabilistic or non-probabilistic sampling techniques?

If you intend to apply probability based statistical tests on the sample, you must use probabilistic sampling techniques.

**Summary:**

**Sampling techniques may be broadly classified as Probabilistic or Non-Probabilistic techniques.**

**Probabilistic techniques usually involve the term “Random”.**

**Probabilistic means the probability (or likelihood) of selection is known, and is non-zero.**

**Non-Probabilistic techniques are usually employed when either the probability of selection is not known, or the probability of selection is zero for some members of the population.**

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