Independent Variable
The independent variable is defined on the Specifications tab
by its name or column position. These controls will automatically be set
when the SEER*Stat Export Dictionary is loaded. If you created your data
file with software other than SEER*Stat you must set these controls.
The
independent variable or covariate is typically the year of diagnosis or
year of death. Joinpoint also allows the independent variable to
have non-integer or negative values. You are limited to a 40-character
label for the independent variable name.
The
“Shift Data Points by” option allows all the values for the independent
variable to be shifted up by a fixed value. This
is done by simply entering a value for “Shift Data Points by”. For
example, if your independent variable is years (inputted as 1975, 1976,…),
but you would like these points to represented on the graph at the midpoint
of the years (1975.5, 1976.5, …), then you would enter the value 0.5 for
this option. Shifting the
data points will change the location of the joinpoints and the intercepts but will not change the
slopes
or APCs.
The
effect of entering a value for this option can be seen by clicking on
“Define” for the Independent Variable. Note:
this value
cannot be greater than the maximum interval between data points.
One reason for employing this option would be because each
data point represents a summary of data collected over a time interval.
For example, cancer incidence or mortality data
is often collected over the course of a year, and is usually entered as
a whole year value, e.g. 1990, 1991. Instead,
one may want to shift by half a year so that the data point is represented
as the midpoint of the interval, e.g. 1990.5, 1991.5).
This is especially important if joinpoints are
allowed to occur at places other than the data points (either in continuous
time using Hudson’s algorithm, or using a grid search where grid points
are allowed between data points). If
the data points are not shifted, the results may be counterintuitive.
For example, without a shift, a joinpoint located
at 1989.50 represents the beginning of 1990, and 1990.49 represents the
end of 1990. If the points
are shifted by half a year, then 1990.00 represents the start of the year
and 1990.99 represents the end of the year.