THE LECTURE COVERS PROBABILITY THEORY, RANDOM SELECTION REPRESENTATIVE SAMPLES

1. SOC 340 LECTURE: WK 2
Representativeness & Randomness*
*Source:
Creswell, J. W. (2003). Research design: Qualitative, quantitative & mixed method. Thousand Oaks, CA: Sage
Babbie, E. (2014). The basics of social research. Belmont, CA: Sage Publishing 1

2. PROBABILTY THEORY OF SAMPLING
WHAT IS PROBABILITY?
PROBABILITY IS THE MEASURE OF THE LIKELIHOOD THAT AN EVENT WILL OCCUR. PROBABILITY IS
QUANTIFIED AS A NUMBER BETWEEN 0 AND 1, WHERE, LOOSELY SPEAKING, 0 INDICATES IMPOSSIBILITY
AND 1 INDICATES CERTAINTY. THE HIGHER THE PROBABILITY OF AN EVENT, THE MORE LIKELY IT IS THAT THE
EVENT WILL OCCUR. A SIMPLE EXAMPLE IS THE TOSSING OF A FAIR (UNBIASED) COIN.
WHAT IS PROBABILITY SAMPLING?
PROBABILITY SAMPLING DESCRIBES THE PROCESS OF SELECTING SAMPLES IN ACCORDANCE WITH
PROBABILITY THEORY WHICH TYPICALLY INCLUDE A RANDOM PROCESS FOR SELECTION. TYPES OF
PROBABILITY SAMPLING THAT WE WILL BE CONCERNED WITH ARE:
SIMPLE RANDOM SAMPLING
SYSTEMATIC RANDOM SAMPLING
WHAT IS NON-PROBABILITY SAMPLING?
ANY SAMPLING SELECTION PROCESS THAT DOES NOT CONFORM TO PROBABILITY THEORY. TYPES OF
NONPROBABILITY SAMPLING THAT WE WILL BE CONCERNED WITH ARE:
PURPOSIVE SAMPLING
SNOWBALL SAMPLING
AVAILABLE RESPONDENTS SAMPLING
QUOTA SAMPLING
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3. SAMPLING BIAS
REPRESENTATIVENESS
PURPOSE OF THE SAMPLING PROCESS:
TO PROVIDE INFORMATION ABOUT A TOTAL POPULATION BY SELECTING A SAMPLE OF THAT POPULATION THAT
CONTAINS THE SAME VARIATIONS AS THE TOTAL POPULATION. IN OTHER WORDS, THE SAMPLE MUST BE
REPRESENTATIVE OF THE POPULATION. FOR INSTANCE, IF THE TOTAL PUPULATION IS SPLIT 50/50 BETWEEN
GENDER TYPES, THE SAMPLE SHOULD ALSO BE SPLIT IN CLOSE TO THE SAME WAY, IF NOT EXACTLY.
BIAS:
RESULTS FROM INADEQUATE PREPARATION OF THE SAMPLE SELECTION, WHICH LEADES TO A LACK OF
REPRESENTATIVENESS.
REPRESENTATIVENESS:
THE QUALITY OF THE SAMPLE HAVING THE SAME DISTRIBUTION OF CHARACTERISTICS AS THE TOTAL
POPULATION. REPRESENTATIVENESS IS IMPROVED BY PROBABILITY SAMPLING & CAN BE THE BASIS FOR
GENERALIZING THE RESULTS OF THE RESEARCH STUDY.
EQUAL PROBABILITY OF SELECTION:
A METHOD THAT ENSURES EACH MEMBER OF THE TOTAL POPULATION HAS THE SAME CHANCE OF BEING
SELECTED AS A MEMBER OF THE SAMPLE GROUP
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4. PROBABILITY THEORY
PROBABILITY THEORY:
A MATHEMATICAL PROCESS THAT ENSURES RESEARCHERS WILL SELECT SAMPLE POPULATIONS THAT
ARE REPRESENTATIVE OF THE TOTAL POPULATION FROM WHICH THE SAMPLE IS DERIVED. IT ALSO
ENABLES RESEARCHERS TO STATISTICALLY ANALYZE THE RESULTS OF THE RESEARCH STUDY & TO
APPLY THOSE RESULTS TO THE TOTAL POPULATION (I.E., GENERALIZE THE RESULTS.
RANDOM SELECTION:
IS THE PROCESS BY WHICH EACH ELEMENT (MEMBER OF THE TOTAL POPULATION) HAS AN EQUAL
CHANCE OF BEING SELECTED INTO THE SAMPLE POPULATION. FOR EXAMPLE: FLIPPING A COIN 100
TIMES. SINCE THERE ARE ONLY TWO POSSIBILITIES, HEADS OR TAILS, THERE IS AN EQUAL CHANCE
THAT ONE OR THE OTHER WILL BE THE RSULT OF A FLIP.
REASONS FOR USING RANDOM SLECTION:
1. IT REDUCES THE CHANCE OF UNCONSCIOUS BIAS
2. IT CONFORMS TO PROBABILITY THEORY & IMPROVES THE ACCURACY OF THE SAMPLE
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5. KEY TERMS
ELEMENT:
THE UNIT OF WHICH THE POPULATION (E.G.; WILMU STUDENTS) IS COMPRISED & WHICH IS
SELECTED FOR THE SAMPLE
TOTAL POPULATION (ALSO CALLED THE STUDY POPULATION):
THE AGGREGATION OF ALL THE ELEMENTS BEING STUDIED (E.G.; WILMU STUDENTS)
SAMPLE POPULATION:
A SUB POPULATION OF THE TOTAL POPULATION RANDOMLY SELECTED THAT IS REPRESENTATIVE OF THE
TOTAL POPULATION
SAMPLING UNIT:
THE ELEMENT OF SET OF ELEMENTS SELECTED IN SOME STAGE OF SAMPLING
PARAMETER:
THE SUMMARY DESCRIPTION OF A GIVEN VARIABLE IN A POPULATION
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