Numbers Needed for Change

This function computes the Numbers Needed for Change, and shows a visualisation to illustrate them. nnt is an alias for nnc.

nnc(d = NULL, cer = NULL, r = 1, n = NULL,
    threshold = NULL, mean = 0, sd = 1,
    poweredFor = NULL, thresholdSensitivity = NULL,
    eventDesirable = TRUE, eventIfHigher = TRUE,
    conf.level=.95,
    d.ci = NULL, cer.ci = NULL, r.ci = NULL,
    d.n = NULL, cer.n = NULL, r.n = NULL,
    plot = TRUE, returnPlot = TRUE, silent = FALSE)

Arguments

d	The value of Cohen's d.
cer	The Control Event Rate.
r	The correlation between the determinant and behavior (for mediated Numbers Needed for Change).
n	The sample size.
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 value, used to draw the plot, or, if no CER is provided but instead the threshold value, to compute the CER.
sd	The standard deviation, used to draw the plot (and to compute the CER if a threshold value is supplied instead of the CER).
poweredFor	The Cohen's d value for which the study was powered. This expected Cohen's d value can be used to compute the threshold, which then in turn is used to compute the CER. To use this approach, also specify the mean and the standard deviation.
thresholdSensitivity	This argument can be used to provide a vector of potential threshold values, each of which is used to compute an NNC. This enables easy inspection of whether the value chosen as threshold matters much for the NNC.
eventDesirable	Whether an event is desirable or undesirable.
eventIfHigher	Whether scores above or below the threshold are considered 'an event'.
conf.level	The confidence level of the confidence interval.
d.ci	Instead of providing a point estimate for Cohen's d, a confidence interval can be provided.
cer.ci	Instead of providing a point estimate for the Control Event Rate, a confidence interval can be provided.
r.ci	Instead of providing a point estimate for the correlation, a confidence interval can be provided.
d.n	In addition to providing a point estimate for Cohen's d, a sample size can be provided; if it is, the confidence interval is computed.
cer.n	In addition to providing a point estimate for the Control Event Rate, a sample size can be provided; if it is, the confidence interval is computed.
r.n	In addition to providing a point estimate for the correlation, a sample size can be provided; if it is, the confidence interval is computed.
plot	Whether to generate and show the plot.
returnPlot	Whether to return the plot (as an attribute), or to only display it.
silent	Whether to suppress notifications.

Details

This function computes the Numbers Needed for Change. See Gruijters & Peters (2017) for details.

Value

The Numbers Needed for Change (NNC), potentially with a plot visualising the NNC in an attribute.

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.

Examples

### Simple example
nnc(d=.4, cer=.3);
#> 
#> Numbers Needed for Change: 7
#> 
#> (Based on a Control Event Rate (CER) of .3, an Experimental Event Rate (EER) of .45 and a Cohen's d of 0.4.)

### Or for a scenario where events are undesirable, and the 
### intervention effective (therefore having a negative value for d):
nnc(d=-.4, cer=.3, eventDesirable=FALSE);
#> 
#> Numbers Needed for Change: 9
#> 
#> (Based on a Control Event Rate (CER) of .3, an Experimental Event Rate (EER) of .18 and a Cohen's d of -0.4.)