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Lecture 3 Descriptives & Graphing Lecturer: James Neill. Research Methods & Design in Psychology. Overview. Univariate descriptives & graphs Non-parametric vs. parametric Non-normal distributions Properties of normal distributions Graphing relations b/w 2 and 3 variables.

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lecture 3 descriptives graphing lecturer james neill
Lecture 3

Descriptives & Graphing

Lecturer: James Neill

Research Methods & Design in Psychology

Univariate descriptives & graphs

Non-parametric vs. parametric

Non-normal distributions

Properties of normal distributions

Graphing relations b/w 2 and 3 variables

empirical approach to research
Empirical Approach to Research

A positivistic approach ASSUMES:

the world is made up of bits of data which can be ‘measured’, ‘recorded’, & ‘analysed’

Interpretation of data can lead to valid insights about how people think, feel and behave

what do we want to describe
What do we want to Describe?

Distributional properties of variables:

Central tendency(ies)


Spread / Dispersion

basic univariate descriptive statistics
Basic Univariate Descriptive Statistics

Central tendency





  • Interquartile Range
  • Range
  • Standard Deviation
  • Variance


  • Skewness
  • Kurtosis
basic univariate graphs
Basic Univariate Graphs

Bar Graph – Pie Chart

Stem & Leaf Plot



measures of central tendency
Measures of Central Tendency

Statistics to represent the ‘centre’ of a distribution

Mode (most frequent)

Median (50th percentile)

Mean (average)

Choice of measure dependent on

Type of data

Shape of distribution (esp. skewness)

measures of central tendency8
Measures of Central Tendency

















measures of dispersion
Measures of Dispersion

Measures of deviation from the central tendency

Non-parametric / non-normal:range, percentiles, min, max

Parametric:SD & properties of the normal distribution

measures of dispersion10
Measures of Dispersion

Range, Min/Max














describing nominal data
Describing Nominal Data


Most frequent?

Least frequent?


Bar graphs

Examine comparative heights of bars – shape is arbitrary

Consider whether to use freqs or %s


Number of individuals obtaining each score on a variable

Frequency tables

graphically (bar chart, pie chart)

Can also present as %


Most common score - highest point in a distribution

Suitable for all types of data including nominal (may not be useful for ratio)

Before using, check frequencies and bar graph to see whether it is an accurate and useful statistic.

describing ordinal data
Describing Ordinal Data

Conveys order but not distance (e.g., ranks)

Descriptives as for nominal (i.e., frequencies, mode)

Also maybe median – if accurate/useful

Maybe IQR, min. & max.

Bar graphs, pie charts, & stem-&-leaf plots

stem leaf plot
Stem & Leaf Plot
  • Useful for ordinal, interval and ratio data
  • Alternative to histogram
box whisker
Box & whisker
  • Useful for interval and ratio data
  • Represents min. max, median and quartiles
describing interval data
Describing Interval Data

Conveys order and distance, but no true zero (0 pt is arbitrary).

Interval data is discrete, but is often treated as ratio/continuous (especially for > 5 intervals)

Distribution (shape)

Central tendency (mode, median)

Dispersion (min, max, range)

Can also use M & SD if treating as continuous

describing ratio data
Describing Ratio Data

Numbers convey order and distance, true zero point - can talk meaningfully about ratios.


Distribution (shape – skewness, kurtosis)

Central tendency (median, mean)

Dispersion (min, max, range, SD)

the four moments of a normal distribution
The Four Moments of a Normal Distribution






the four moments of a normal distribution28
The Four Moments of a Normal Distribution

Four mathematical qualities (parameters) allow one to describe a continuous distribution which as least roughly follows a bell curve shape:

  • 1st = mean (central tendency)
  • 2nd = SD (dispersion)
  • 3rd = skewness (lean / tail)
  • 4th = kurtosis (peakedness / flattness)
mean 1 st moment
Mean (1st moment )
  • Average score
  • Mean =  X / N
  • Use for ratio data or interval (if treating it as continuous).
  • Influenced by extreme scores (outliers)
standard deviation 2 nd moment
Standard Deviation (2nd moment )
  • SD = square root of Variance

=  (X - X)2

N – 1

  • Standard Error (SE) = SD / square root of N
skewness 3 rd moment
Skewness (3rd moment )
  • Lean of distribution
  • +ve = tail to right
  • -ve = tail to left
  • Can be caused by an outlier
  • Can be caused by ceiling or floor effects
  • Can be accurate (e.g., the number of cars owned per person)
skewness 3 rd moment32
Skewness (3rd moment )
  • Negative skew
  • Positive skew
kurtosis 4 th moment
Kurtosis (4th moment )
  • Flatness or peakedness of distribution
  • +ve = peaked
  • -ve = flattened
  • Be aware that by altering the X and Y axis, any distribution can be made to look more peaked or more flat – so add a normal curve to the histogram to help judge kurtosis
kurtosis 4 th moment36
Kurtosis (4th moment )

Red = Positive (leptokurtic)

Blue = negative (platykurtic)

key areas under the curve for normal distributions
Key Areas under the Curve for Normal Distributions
  • For normal distributions, approx. +/- 1 SD = 68%+/- 2 SD ~ 95%+/- 3 SD ~ 99.9%
types of non normal distribution
Types of Non-normal Distribution
  • Bi-modal
  • Multi-modal
  • Positively skewed
  • Negatively skewed
  • Flat (platykurtic)
  • Peaked (leptokurtic)
rules of thumb in judging severity of skewness kurtosis
Rules of Thumb in Judging Severity of Skewness & Kurtosis
  • View histogram with normal curve
  • Deal with outliers
  • Skewness / kurtosis <-1 or >1
  • Skewness / kurtosis significance tests
skewed distributions the mode median mean
Skewed Distributions& the Mode, Median & Mean

+vely skewed mode < median < mean

Symmetrical (normal) mean = median = mode

-vely skewed mean < median < mode

edward tufte
Edward Tufte


Reveal data

Communicate complex ideas with clarity, precision, and efficiency

tufte s guidelines 1
Tufte's Guidelines 1

Show the data

Substance rather than method

Avoid distortion

Present many numbers in a small space

Make large data sets coherent

tufte s guidelines 2
Tufte's Guidelines 2
  • Encourage eye to make comparisons
  • Reveal data at several levels
  • Purpose: Description, exploration, tabulation, decoration
  • Closely integrated with statistical and verbal descriptions
tufte s graphical integrity 1
Tufte’s Graphical Integrity 1

Some lapses intentional, some not

Lie Factor = size of effect in graph size of effect in data

Misleading uses of area

Misleading uses of perspective

Leaving out important context

Lack of taste and aesthetics

tufte s graphical integrity 2
Tufte's Graphical Integrity 2

Trade-off between amount of information, simplicity, and accuracy

“It is often hard to judge what users will find intuitive and how [a visualization] will support a particular task” (Tweedie et al)

  • Presenting Data – Statistics Glossary v1.1 - http://www.cas.lancs.ac.uk/glossary_v1.1/presdata.html
  • A Periodic Table of Visualisation Methods - http://www.visual-literacy.org/periodic_table/periodic_table.html
  • Gallery of Data Visualization
  • Univariate Data Analysis – The Best & Worst of Statistical Graphs - http://www.csulb.edu/~msaintg/ppa696/696uni.htm
  • Pitfalls of Data Analysis – http://www.vims.edu/~david/pitfalls/pitfalls.htm
  • Statistics for the Life Sciences –http://www.math.sfu.ca/~cschwarz/Stat-301/Handouts/Handouts.html