Hints for better data display

• Why data display?

• Good ideas for future presentations

• Data display is never perfect

• Judges

My approach today

• I will show good and bad examples

• As I proceed ask yourself how each figure could be improved.

• I will also list some rules that you could use as guidelines.

Goal of display?

• Strive to promote evaluation of objectives/hypotheses

• Clearly depict relationships

• Figures are usually preferable to tables

• Use tables if the exact values are important

• Very simple comparisons don’t require a figure or table

Rule 1: Avoid Unnecessary Complexity

Availability of Brood and Nesting Habitat in Each Managed Area in 2006

0%10%20%30%40%50%60%70%80%90%

100%

Managment Areas

Per

cen

t C

lass

ifie

d

Classifiedas "Brood"

Classifiedas "Nest"

Rule 2: Avoid Jargon and be careful with abbreviations

• You want your audience to have as little trouble following you as possible

• By the way, what is wrong with this slide?

Loss of PIT tags by Desmognathus brimleyorum in SMZ’s that varied from 10 to 200 M in width

Width of SMZ

Loss of tags by Ouachita dusky salamanders in relation to habitat width

Width of Streamside Management Zone

Rule 3: Don’t display raw data

X Y

22 17 21 17 15 17 19 11 18 12 16 11 23 18 25 18 14 17 11 12 21 15

(r2=0.24 P=0.6)

Flight speed of quail encumbered and unencumbered by radio transmitters

Without transmitter 22 MPH With transmitter 17 MPHWithout transmitter 21 MPH With transmitter 17 MPHWithout transmitter 17 MPH With transmitter 20 MPHWithout transmitter 15 MPH With transmitter 17 MPHWithout transmitter 19 MPH With transmitter 11 MPHWithout transmitter 18 MPH With transmitter 12 MPHWithout transmitter 16 MPH With transmitter 11 MPHWithout transmitter 23 MPH With transmitter 18 MPHWithout transmitter 25 MPH With transmitter 18 MPHWithout transmitter 14 MPH With transmitter 17 MPHWithout transmitter 11 MPH With transmitter 12 MPHWithout transmitter 21 MPH With transmitter 15 MPH

Unencumbered Flight speed Encumbered Flight speed

Table 1. What was wrong?

1. Conclusion difficult to see 2. Too messy

3. Units?4. Font too small in some places

Flight speed of 11 quail encumbered and unencumbered by radio transmitters

22 17 21 17 15 17 19 11 18 12 16 11 23 18 25 18 14 17 11 12 21 15

Unencumbered Speed KPH Encumbered speed KPH

Same data in a figure and a table

0102030

Fli

ght

Spee

d

Mean Flight Speed of Quail With and Without Transmitters.

With Transmitters Without Transmitters

15.4 KPH 18.5 KPH

W Trans W/O Trans

Mean Flight Speed of Quail With and Without Transmitters

Which was Better?

• In this case I wouldn’t use a figure or a table.

• Average flight speed of unencombered quail was 3.1 Km/hr faster than Quail encumbered by transmitters.

• The difference is statistically significant (t=2.24, P=0.02 DF=10).

Same data; what is wrong with this?

Same data; what is wrong with this?

• Axes not labeled

• Y-axis too messy

• Stats? F statistic; p-value and r2

Rule 4: Don’t cut and paste figures from publications

Male and Female American Kestrels

Male and Female European Kestrels

More Complex Data

Proportion of arthropods in the diets of male and female North American and European Kestrels

Patterns are more easily interpreted when data are displayed in figures

• But:

• Make sure the figure is easy to interpret

• The best figures are able to stand alone, i.e., they don’t need any help from the presenter

• So, for every figure ask yourself: “Is there a better way to display the data”?

Kestrel data poorly displayed; interaction difficult to see

% In

sect

s in

Die

t

Winter Spring

Stacked bars are almost never your best choice

Winter Spring

% In

sect

s in

Die

t

The interaction shows up easily here, could it be better?

Winter Spring

% In

sect

s in

Die

t

For bar graphs

• Put bars together to illustrate the conclusions you are trying to display.

• Make sure that your eye doesn’t have to travel back and forth across the figure in order to draw a conclusion

For example

• Given 4 breast cancer cell lines and 4 chemotherapies

• If you are interested in how the different cell lines respond to the treatments:

Group the data for each cell line together%

Apo

ptos

is

However,

• If you are interested in the efficacy of the different treatments among cell lines:

Group the data for the different treatments%

Apo

ptos

is

What is missing?%

Apo

ptos

is

Rule 3: Display Error Bars

• If you have replication of continuously distributed data you should display error bars.

• Standard error, 95% confidence intervals and standard deviation are commonly used.

• Don’t chose arbitrarily, know your error bars and what they mean

Rule 5: Display Error Bars

Rule 6: do not connect points in a scatter plot unless they are related

Make sure the dependent and independent variables are on the proper axis

What else is wrong with this figure?

Species richness

Too many lines: limit your line graphs to 3-4 lines; Other problems?

Split into two graphs: What else is wrong?

Solution Present two graphs

Summary of common mistakes in data display

• Too many bars or lines are included

• Lines connect independent data points

• Raw data are displayed

• Error bars are absent

More common mistakes in data display

• Tables are used when a figure is more appropriate

• Figures don’t illustrate anything

• Figures are too complicated

• The “wrong” figure type is used

Statistics and Study Design

• Most of the time fairly simple statistical tests will suffice:

• For categorical comparisons and proportions: Chi-squared

• For two continuously distributed variables in a linear relationship: linear regression

• For comparison of two means: t-test

• For comparison of three means: ANOVA

Be careful

• All of the above require independent and randomly drawn samples (one exception)

• What if you take a before and after measurement on a set of subjects?

• What if you take multiple measurements on each member of a sample population?

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