Visualising Numbers Needed for Change

These functions can be used to visualise Numbers Needed for Change. erDataSeq is a helper function to generate an Event Rate Data Sequence, and it uses convert.threshold.to.er and convert.er.to.threshold to convert thresholds to event rates and vice versa.

erDataSeq(er = NULL, threshold = NULL,
          mean = NULL, sd = NULL,
          eventIfHigher = TRUE,
          pRange = c(1e-06, 0.99999),
          xStep = 0.01)

ggNNC(cerDataSeq, d = NULL, eventDesirable = TRUE,
      r = 1, xlab = "Continuous outcome",
      plotTitle = c("Numbers Needed for Change = ", ""),
      theme = theme_bw(), lineSize = 1,
      cerColor = "#EBF2F8", eerColor = "#172F47",
      cerLineColor = "#888888", eerLineColor = "#000000",
      dArrowColor = "#000000", cerAlpha = 0.66,
      eerAlpha = 0.66, xLim = NULL,
      xLimAutoDensityTolerance = 0.001,
      showLegend = TRUE, verticalLineColor = "#172F47",
      desirableColor = "#00FF00", desirableAlpha = 0.2,
      undesirableColor = "#FF0000", undesirableAlpha = 0.2,
      desirableTextColor = "#009900",
      undesirableTextColor = "#990000",
      dArrowDistance = 0.04 * max(cerDataSeq$density),
      dLabelDistance = 0.08 * max(cerDataSeq$density))

convert.threshold.to.er(threshold, mean, sd,
                        eventIfHigher = TRUE,
                        pdist = pnorm)

convert.er.to.threshold(er, mean, sd,
                        eventIfHigher = TRUE,
                        qdist = qnorm)

Arguments

er

Event rate to visualise (or convert).
threshold

If the event rate is not available, a threshold value can be specified instead, which is then used in conjunction with the mean (mean) and standard deviation (sd) and assuming a normal distribution to compute the event rate.
mean

The mean of the control group distribution.
sd

The standard deviation (of the control distribution, but assumed to be the same for both distributions).
eventIfHigher

Whether scores above or below the threshold are considered 'an event'.
pRange

The range of probabilities for which to so the distribution.
xStep

Precision of the drawn distribution; higher values mean lower precision/granularity/resolution.
cerDataSeq

The cerDataSeq object.
d

The value of Cohen's d.
eventDesirable

Whether an event is desirable or undesirable.
r

The correlation between the determinant and behavior (for mediated NNC's).
xlab

The label to display for the X axis.
plotTitle

The title of the plot; either one character value, this value if used; if two, they are considered a prefix and suffix to be pre/appended to the NNC value.
theme

The theme to use for the plot.
lineSize

The thickness of the lines in the plot.
cerColor

The color to use for the event rate portion of the control group distribution.
eerColor

The color to use for the event rate portion of the experimental group distribution.
cerLineColor

The line color to use for the control group distribution.
eerLineColor

The line color to use for the experimental group distribution.
dArrowColor

The color of the arrow to show the effect size.
cerAlpha

The alpha value (transparency) to use for the control group distribution.
eerAlpha

The alpha value (transparency) to use for the control group distribution.
xLim

This can be used to manually specify the limits for the X axis; if NULL, sensible limits will be derived using xLimAutoDensityTolerance.
xLimAutoDensityTolerance

If xLim is NULL, the limits will be set where the density falls below this proportion of its maximum value.
showLegend

Whether to show the legend (only if showing two distributions).
verticalLineColor

The color of the vertical line used to indicate the threshold.
desirableColor

The color for the desirable portion of the X axis.
desirableAlpha

The alpha for the desirable portion of the X axis.
undesirableColor

The color for the undesirable portion of the X axis.
undesirableAlpha

The color for the undesirable portion of the X axis.
desirableTextColor

The color for the text to indicate the desirable portion of the X axis.
undesirableTextColor

The color for the text to indicate the undesirable portion of the X axis.
dArrowDistance

The distance of the effect size arrow from the top of the distributions.
dLabelDistance

The distance of the effect size label from the top of the distributions.
pdist, qdist

Distributions to use when converting thresholds to event rates and vice versa; defaults to the normal distribution.

Details

These functions are used by nnc to show the distributions, and event rates. They probably won't be used much on their own.

Value

erDataSeq returns a data sequence; ggNNC a ggplot.

References

Gruijters, S. L. K., & Peters, G.-J. Y. (2017). Introducing the Numbers Needed for Change (NNC): A practical measure of effect size for intervention research.

See also

nnc

Examples

### Show distribution for an event rate value of 125
ggNNC(erDataSeq(threshold=125, mean=90, sd=30));

### If the event occurs under the threshold instead of
### above it
ggNNC(erDataSeq(threshold=125, mean=90, sd=30,
      eventIfHigher = FALSE));

### ... And for undesirable events (note how
### desirability is an argument for ggNNC, whereas
### whether an event occurs 'above' or 'below' the
### threshold is an argument for erDataSeq):
ggNNC(erDataSeq(threshold=125, mean=90, sd=30,
                eventIfHigher = FALSE),
      eventDesirable = FALSE);

### Show event rate for both experimental and
### control conditions, and show the numbers
### needed for change
ggNNC(erDataSeq(threshold=125, mean=90, sd=30), d=.5);

### Illustration of how even with very large effect
### sizes, if the control event rate is very high,
### you'll still need a high number of NNC
ggNNC(erDataSeq(er=.9), d=1);