The Kaplan-Meier Tab

The Kaplan-Meier tab estimates the survival function nonparametrically and compares groups using RMST (formulation). Visually assess differences in survival between groups and quantify them using restricted mean survival time. See Survival Analysis Fundamentals for the mathematical background.

To estimate the effect of covariates on hazard, use The Cox Regression Tab.

Data Requirements

Kaplan-Meier analysis requires two variables:

• Time variable: Time to event (numeric)
• Event variable: Indicates whether the event occurred. The following formats are supported:
  • int64: 1 = event, 0 = censored
  • Boolean: true = event, false = censored

float64 columns cannot be selected as the event variable. If a column stores 0/1 values as decimals, convert it to int64 with The Convert Column Types Tab.

See Survival Analysis Fundamentals for how censoring is handled. MIDAS only supports right censoring. Left censoring, interval censoring, and competing risks are not supported.

Basic Usage

1. Select Analysis > Survival Analysis > Kaplan-Meier... from the menu bar
2. Select the Time Variable
3. Select the Event Variable
4. Optionally select a Group Variable for group comparison
5. Click Run Analysis

The Group Variable dropdown lists only columns whose scale of measurement is Nominal or Ordinal. A column encoded as 0/1 integers, such as anaemia, is read as numeric and cannot be selected as is. Change its scale of measurement to Nominal in the Data Table tab first. See the Kaplan-Meier tutorial for the steps.

Understanding Results

Survival Curve

Plots survival probability $S(t)$ against time. Displayed as a step function that decreases at each event time. Censoring times are marked with a + symbol on the curve. A + mark on a flat segment indicates that subjects were lost to follow-up during that interval. A pointwise confidence band (95% by default) is shown. A pointwise band consists of individual intervals at each time point and does not guarantee simultaneous coverage of the entire curve, computed using the log transformation method (details).

Adjust the confidence level with the Confidence Level input.

Summary Statistics

Number at Risk

Shows the risk set size (the number of subjects who have not yet experienced the event and have not been censored) at each time point.

RMST (Restricted Mean Survival Time)

The average survival time estimated as the area under the Kaplan-Meier curve from 0 to a restriction time $\tau$ (formulation). Per-group RMST with SE and confidence interval is displayed.

Restriction Time $\tau$

RMST is the area under the KM curve up to $\tau$, so the choice of $\tau$ affects the result. The default is the upper bound of the range commonly observed across all groups (the smallest of each group's maximum observed time). Change it with the RMST Restriction Time input. In the interval after the last observed event, the integration assumes the KM curve stays at its last value (this also happens within the maximum observed time when only censoring follows the last event). The longer this interval, the more the uncertainty of RMST may be underestimated.

Group Differences

When a Group Variable is specified and there are two or more groups, pairwise RMST differences and their confidence intervals are displayed. For three or more groups, the per-pair confidence intervals are unadjusted for multiplicity.

Notes

• Rows with missing values in the time or event variable are automatically excluded (listwise deletion; see Missing Data Mechanisms for validity conditions). When rows are excluded, the results show the number of excluded rows as "N rows excluded due to missing values."

Adding to Reports

Click Add to Report to add the survival curve to a report.

See also

• Survival Analysis Fundamentals - Mathematical background of time-to-event data, Kaplan-Meier, and Cox model
• The Cox Regression Tab - Estimating the effect of covariates on hazard
• Tutorial: Kaplan-Meier Analysis - A practical example with sample data

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