W3 - Descriptive Statistics Flashcards
Define nominal scale
Subjects grouped into mutually exclusive categories.
Examples of nominal scale
Eye colour
Year of birth
Define ordinal scales
Ranks subjects/scores in order.
Doesn’t indicate how much better 1 score is to another.
Example of ordinal scales
University rankings
Define interval scales
Equal units or intervals between data points on a scale, but there’s no zero point.
Example of interval scales
Temperature
Define ratio scales
Equal units of measurement
Established 0 point.
Example of ratio scales
Mass
Length
Height
What are measures of central tendency used for?
To summarise or provide meaning to a whole dataset
To provide a ‘central’ value
What doesn’t central tendency show?
Distribution of scores
What is the SD?
Avg distance away from mean
What allows you to summarise data?
Measures of central tendency + dispersion
What graph would you make if you had a lot of data?
Frequency distribution plot/graph
Properties that all normally distributed data show
Symmetrical
Equal mean, median + mode
What do Z-scores do?
Describes a values relationship to the mean of a group of values.
How is the z-score measured?
In terms of SD from the mean
Equation to calculate z-score
(X - mean) / SD
Where would z-score of 0 be on a normal distribution curve?
In the middle of the bell curve
What do z-scores + normal distribution curve allow you to identify?
Ind. score rel. to pop
Probability of scoring a given z-score or higher/lower
What score is needed to be in a given % of the pop
What are the important z-scores?
- 96
- 58
- 29
Z-score
1.96
95% of scores fall w/in 1.96 Z scores or SDs above/below mean
Z-score
2.58
99% of scores fall w/in 2.58 Z scores or SDs above/below mean
Z-score
3.29
99.9% of scores fall w/in 3.29 Z scores or SDs above/below mean
Type of deviation from normality
Skewness
A measure of lateral deviation from normality
Are skewed distributions symmetrical?
What are they instead?
NO
Most scores are high/low with small % of scores away from the majority
+ively skewed
“Tail” of distribution positioned at upper end of score continuum
-ively skewed
“Tail” of distribution positioned at lower end of score continuum
Type of deviation from normality
Kurtosis
Measure of vertical deviation from normality
What are the distribution curves under kurtosis (type of deviation from normality)
Leptokurtic
Platykurtic
KURTOSIS
Leptokurtic curve
More peaked than a normal distribution curve
KURTOSIS
Platykurtic curve
More flat than a normal distribution curve
KURTOSIS
What is meant by mesokurtic
Level/Distribution of what you would expect from the normal distribution
What is a good guide for the coefficient of skewness?
-1.0 - 1.0
What is a good guide for the coefficient of kurtosis?
-1.0 - 2.0
What are the measures of central tendencies?
Mode
Mean
Median
When would you use the mode
When data is categorical
When would you use the mean?
If you want the total
or
If distribution isn’t skewed
When would you use the median?
If distribution is skewed
Define a predictor variable
Variable thought to predict an outcome variable.
(Like the independent variable).
What can variables be split into?
Categorical
Continuous
What comes under categorical variables?
Binary variables
Nominal variables
Ordinal variables
What comes under continuous variables?
Interval variables
Ratio variables
Define criterion validity
Establishing that an instrument measures what it claims to measure
- by comparison to objective criteria.
What is said to assess concurrent validity?
Comparing a new test to an existing one to see if they produce similar results.
What is said to assess predictive validity?
When data from new instrument is used to predict observations at a later date.
