A survival follow-up interval is a constant time period used to examine survival duration for cases included in a survival analysis. Every survival analysis has a constant number and length of survival intervals. The number of observation intervals and the months per interval are universal parameters that may be defined on the **Survival Parameters** tab. The system provides the ability to create groupings of regular intervals called *special intervals* or *conditional survival* intervals. A z-test interval may also be defined; to set it, see **Display** Options for a **Survival** Session.

**Note**: Starting with SEER*Stat version 6.0, the default values for number of intervals and months per interval were changed from five intervals of 12 months each to 60 intervals of one month each. Saved sessions from previous versions of the software will retain their saved values. Although the old and new defaults both span five years, cumulative survival based on them will differ slightly due to the different lengths of the intervals.

## Number of Intervals

The **Number** of intervals indicates the maximum number of survival intervals to be displayed in the output survival tables. The default is 60 intervals (five years, if you use the default of one-month intervals). To change the number of intervals in your analysis, either type in the number of intervals, or use the increment or decrement buttons.

When you change the number of intervals, if you are generating **Cumulative Summary** tables or **Special Intervals/Conditional Survival**, you should update the cumulative summary and special intervals/conditional survival intervals accordingly.

When **Period Survival **is used, this control will be unavailable. This is due to the fact that when **Period Survival** is selected, the number of survival intervals is based on the **Maximum Years of Survival** parameter. Also, when Period Survival is specified, the interval duration is automatically set to one month. This default is specific to **Period Survival** and will not be reflected in the **Number** of intervals control.

## Months Per Interval

The **Months Per** box indicates the length of a survival interval in months. The default is one month. To change the length of the intervals in your analysis, either type in the number of months, or use the increment or decrement buttons.

The Kaplan-Meier method of survival calculation requires single-month intervals. If you set this field to a number other than "1" when using the Kaplan-Meier method, SEER*Stat will issue an error message when you try to execute the session.

When you change the number of months per interval, if you are generating Cumulative Summary tables or Special Intervals/Conditional Survival, you should update the cumulative summary and special intervals/conditional survival intervals accordingly.

When **Period Survival **is used, this control will be disabled.

## Number of Years of Period Survival

When **Period Survival** is used, these controls will be available.

### Maximum Years

This parameter defines the maximum number of survival years that will appear in each **Period Survival** table. It is set to five years by default. Please note that **Period Survival** is automatically computed on a monthly interval basis. So if the **Maximum Years** is set to "5", each **Period Survival** table will have up to 60 monthly intervals (five years of survival).

### Years per Cohort

Because **Period Survival** tables are created from extracting specific survival interval statistics from contributing survival tables, this parameter specifies how many years of diagnosis to place into each specific contributing survival table. That is, if this parameter is set to three years and the **Maximum Years** is set to five years, each **Period Table** will require a set of five contributing survival tables (each using cases diagnosed in three-year overlapping chunks). Also, when **Period Survival** is selected, an estimate year value will be automatically assigned. The estimate year is set to one year less than the maximum year of diagnosis for the database. So, if the database you select has diagnosis cases through 2002, the estimate year will be set to 2001, and no 2002 cases will be used for the analysis.

For example, assume that the settings described above are used, and that the estimate year is set to 2001. SEER*Stat will automatically create a user-defined **Year of Diagnosis** variable consisting of the following five categories: 2001-1999, 2000-1998, 1999-1997, 1998-1996, and 1997-1995. This user-defined variable is automatically set to the innermost column variable in order to create the base survival tables needed for the **Period** tables for each existing page variable combination (remember that the row and column variables are considered page variables when creating **Period** and/or **Standard Life** pages in **Survival** sessions). This special **Year of Diagnosis** variable will not be displayed as a column variable on the Table tab; it is added at run-time.

## Period Survival Estimate Years

When **Period Survival** is used, this control will be available. Enter one or more years or ranges of years that are within the time period covered by your database, using a hyphen between the beginning and end of a range, and a comma between non-consecutive years or ranges, e.g. "1980,1985-1990,1992-1994".

Your results matrix will include separate sets of estimates for each of the years specified in this field, arranged as if **Estimate Year** were the last column variable on the **Table** tab.

When you execute the session, SEER*Stat will automatically re-order and consolidate the contents of this field so that years are listed once only and in chronological order. Results will be displayed in the reverse of that order.

## Include 0th Interval

Mark the **Include 0th Interval** checkbox to generate statistics in your output survival table for time zero. The 0th interval will have a survival of one. Some statistics will not be applicable at this point. This option is recommended when you want to graph a survival curve and want the graph to start at time zero.

## Conditional Survival (Special Intervals)

*Special intervals* and *conditional survival* intervals are sets of the regular intervals or specifications for conditional survival. Enter ranges in this box, with hyphens between the beginning and end of a range, and commas between ranges. For a table of 10 yearly intervals, you might enter "6-10", which produces the conditional survival of 10 years, given having already survived 5 years. A special interval is a range of intervals that starts at one (e.g. "1-5"), and thus is not conditional. Special intervals give you the cumulative survival for the upper bound of the range you define. Thus, if you specify a range of "1-5" in this field, you will get the cumulative five-year survival statistics in your results matrix.

To define special intervals or conditional survival intervals in this field, follow these steps.

- Determine which intervals you wish to group into a special interval.

For example, if the interval length is 12 months, to specify five-year intervals which are conditional on the survival of the first five years and the first ten years, you need the intervals from 6 to 10 and 11 to 15.

- Key each interval group into the
**Special Intervals/Conditional Survival**field.

In our example, you would type the following: 6-10,11-15

If you are choosing to display a cumulative summary table, you can mark the Include Special Intervals/Conditional Survival box to include the selected cumulative summary statistics for each of the special intervals in your cumulative summary table.