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Effective Use of Graphs. Annie Herbert Medical Statistician Research & Development Support Unit Salford Royal (Hope) Hospitals NHS Foundation Trust annie.herbert@manchester.ac.uk (0161 720) 2227. Timetable. Outline. Graphs for categorical data Graphs for numerical data Comparing groups

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effective use of graphs

Effective Use of Graphs

Annie Herbert

Medical Statistician

Research & Development Support Unit

Salford Royal (Hope) Hospitals NHS Foundation Trust

annie.herbert@manchester.ac.uk

(0161 720) 2227

outline
Outline
  • Graphs for categorical data
  • Graphs for numerical data
  • Comparing groups
  • Additional graphs (covered in other courses)
  • Final tips & Computer packages
categorical data 1
Categorical Data (1)

Examples:

  • Sex

– Male/Female

  • Blood Group

– A/B/AB/O

  • Employment Status

– Unemployed/Part-time/Full-time

categorical data 2
Categorical Data (2)
  • Record: Frequency (discrete number) per category
  • Summary: Frequency OR

percentage/fraction/proportion

  • Visually:

- Bar Chart - Pie Chart

bar charts adv disadv
Bar Charts – Adv & Disadv
  • Advantages:

- Visually strong.

- Easy to compare between more than one dataset.

  • Disadvantages:
  • Categories can be ‘re-ordered’ to emphasize certain effects.
  • Misleading if not used for counts.
  • Misleading if y-axis not from 0.
bar charts things to consider
Bar Charts – Things to consider:
  • What group differences are you interested in?
  • Frequencies or percentages? If percentage, it’s down to you to specify the totals.
  • Is ‘Other’ a large frequency/percentage?
  • Consider the categories as un-ordered when using a stacked bar chart.
pie charts advantages
Pie Charts – Advantages:
  • Easy to compare categories, are equidistant from each other.
  • Ordering of categories does not emphasize certain effects as badly as stacked bar charts do.
pie charts disadvantages
Pie Charts –Disadvantages:
  • No choice between frequencies and percentages (down to you to specify totals).
  • Cannot put more than one data set into a pie chart.
  • Lose individual values of the data.
  • Limited space: if using more than 5 or 6 categories, chart can look complicated.
numerical data 1
Numerical Data (1)

Examples:

  • Weight
  • Blood Pressure
  • Cholesterol Levels
numerical data 2
Numerical Data (2)
  • Record: Number/Value

(discrete or continuous)

  • Summary: - Mean (SD)

- Median (IQR)

  • Visually:

- Histogram - Box plot - Spread plot

histograms advantages
Histograms – Advantages:
  • Visual display of interval frequencies, easy to compare intervals.
  • Can give an idea of the distribution of the data, e.g. shape, typical value, spread.
histograms disadvantages
Histograms– Disadvantages:
  • Choice of interval width can alter appearance.
  • Individual values lost.
  • One data set per histogram, difficult to compare data sets.
box plot
Box Plot

Extreme Outlier

Upper Quartile

Median

Lower Quartile

Outlier

box plots advantages
Box Plots– Advantages:
  • Defines many summary statistics in one plot.
  • Defines ‘outliers’ explicitly.
  • Can have more than one data set in a plot, so easy to compare data sets:
box plots disadvantages
Box Plots– Disadvantages:
  • More complicated visually than some other types of data plots.
  • Individual values lost.
spread plots 2
Spread Plots (2)
  • Advantages:
  • Can give an idea of the distribution of the data, e.g. shape, typical value, spread.
  • Shows individual values of the data.
  • Can show more than one dataset in a plot.
  • Disadvantages:
  • Not very widely used in journal publications.
  • Doesn’t explicitly summarise statistics or outliers as box plot does.
what information does this give
What information does this give?

Mean ± SE, n ≈ 30 per group

bland altman plots scatter plots
Bland-Altman Plots (scatter plots)

How well do two methods of measurement agree?

final pointers
Final Pointers:
  • Before plotting think about the type of data and what you would like to compare.
  • Show all data rather than summaries where possible.
  • Label axes clearly. Graph should ‘stand alone’.
  • Make sure when comparing groups that outcome on the same scale.
  • Make sure any colours used are sufficiently different from each other, and not red/green.