Software Implementation

Joinpoint

The user supplies the location of the last data point before the coding change occurs. The software automatically locates the discontinuity or "jump" halfway between this last data point and the next data point. The software currently only allows for a single "jump". For example, as reported by Anderson et al. (2001) (PDF), the ICD coding changes from ICD-9 to ICD-10 for classification rules for selecting underlying causes of death. The last year that ICD-9 was used was 1998, and ICD-10 was implemented starting in 1999. If the user had entered annual data, they would enter 1998, and the software would place the "jump" at 1998.5.

Software Input Parameters

• For the jump model, the user supplies:
  • The location of the last data point before the coding change occurs.
  • All other parameters set in the standard joinpoint regression model such as minimum and maximum number of joinpoints, permutation methods, etc.
• For the comparability ratio model, the user supplies:
  • The location of the last data point before the coding change occurs.
  • The size of the comparability ratio. The ratio is limited to [0.01, 100].
  • The variance of the comparability ratio. The variance is limited to [0, 100]. If you use the homoscedastic error option for Joinpoint (i.e. no standard errors) the variance of the comparability ratio will be set to "NA" (not applicable) and will not be able to be changed. If the heteroscedastic error option is used, the user may enter zero if they do not have an estimate of the variance of the comparability ratio, but they are encouraged to enter the variance if they have one.
  • All other parameters set in the standard joinpoint regression model such as minimum and maximum number of joinpoints, permutation methods, etc.
• Restrictions:
  • The jump point must be 2 or more data points from either end of the data.
  • The jump and comparability ratio models can only be run when the Log Transformation is set to Yes.

Software Output

• Both the jump and comparability ratio models will output all the items/statistics produced by the standard joinpoint model.
• The jump model will produce additional statistics related to the jump value and also provide an estimate of the comparability ratio: comparability ratio = exp(jump value). While in almost all situations the comparability ratio and the jump values should both be either statistically different from 0 and 1 respectively (since they are related by a transformation), occasionally, the comparability ratio and jump value do not show the same statistical significance. This minor discrepancy rarely happens. It is due to an approximation from the delta method applied to finding the confidence interval of the comparability ratio of the jump model.

Software Display

The software allows viewing of a graph for each joinpoint model, from the model with the minimum number of joinpoints to the model with the maximum number of joinpoints. The graph will show a jump at the location where the coding change occurs. The software also allows for graphs of different models and/or cohorts to be combined.