L2 basic Stats

Question 1

What does more variance indicate?

Answer 1

A stronger correlation.

Question 2

What is variance?

Answer 2

Variance represents the amount of variability in the data.

• 3 main methods of representing variability.
  1) range -simplest one
  2) Standard deviation (S)
  3) Variance (S2)

Question 3

What is correlation?

Answer 3

• Standardised representation of the association between 2 variables.
• Can range from-1.0 to 1.0
• r symbol for correlation

Question 4

How to calculate variance

Answer 4

• square standard deviation

- square the correlation (coefficient determination)

Question 5

What are composite variables?

Answer 5

• Test scores based on the sum of 2 or more items.
• E.g. The Beck Depression Inventory consists of 21 items
• Items are scored on an ordinal scale from 0 to 3
• Thus, the range of scores possible on the BDI is 0 to 63. (3x21)
• Such ‘sum scores’ are known as ‘composite scores’.

Question 6

The variance of a composite score is a function of:

Answer 6

1. The variance associated with the individual items, and

2. The correlation amongst the items.

Question 7

As the correlation between items increases (and is positive) what also increases?

Answer 7

The magnitude of the corresponding composite variance score.

Question 8

Binary items (dichotomous items)

Answer 8

• you either provide the correct or incorrect answer.
• Responses are scored 0 or 1.
• Frequently used in achievement type tests
• E.g. Exams and intelligence test items

Question 9

Binary items: variance

Answer 9

-The variance of a dichotomously scored item is maximised when half of the people score 1 and the other half score 0.

Question 10

What are the most common psychological test score interpretations?

Answer 10

1) Relative interpretations - based on the analysis if data
2) Abstract interpretations - based on the research which supports the test scores as valid indicators of a psychological construct.

Question 11

Relative interpretations - to interpret an individual’s score, we need to:

Answer 11

-know the mean and SD

Question 12

Raw Scores: Limitations

Answer 12

• Although the mean and SD allow us to interpret a particular score in a relative way, it is not a precise method, unless you’re a human calculator.
• Instead, raw scores can be converted into standardised scores which incorporates information about the mean and SD.
• The most commonly used standardised score is the z-score.

Question 13

Z-scores

Answer 13

• Have a mean of 0 and SD or 1.
• Useful for transforming raw scores into relative scores.
• Shows us the distance from the mean and so frees us from worrying about the units of the original test score.
• Can be used to compare scores across tests that are on different sized units.
• Even if the scales are meaningful, you may want to convert them for the purposes of comparison.

Question 14

Converted Standard Scores

Answer 14

• Z-scores are great but if you didn’t want to deal with negative numbers you can convert them into alternative standard scores.
• To do this rescale the scores so that the converted scores have a different mean and SD.
• The most popular is the T-score
• With T-scores the mean is always 50 and the SD is always 10.
• T-scores are not like t-tests; they’re different.

Question 15

How to create T-scores

Answer 15

1) convert the raw scores unto Z-scores

2) Then, convert the z-scores using the formula: T = z(10) + 50

Question 16

Is this dodgy?

Answer 16

• You may think that applying these conversions is dodgy.
• However you have to remember that most psychological tests yield scores that have no inherent meaning.
• They are measured on ordinal scales.
• They simply yield information in a relative sense.
• How we scale those raw scores does not alter their inherent relative meaning.
• In the case of academic marks, there is one aspect to consider.
• They’re considered “criterion referenced” in a sense.
• That is you need to achieve 50 to pass.

Question 17

What are percentile ranks?

Answer 17

• Percentile ranks indicate the percentage of scores that are below a specific test score.
• another way to represent scores in relative terms.

Question 18

What is a reference sample the same as?

Answer 18

Normative samples.

Question 19

Probability and Non-Probability Sampling

Answer 19

• Probability samples use procedures that ensure a representative sample.
• Random sampling is a type of probability sample, as you would expect a random sample from the population to be representative of the population.