# When to use Nelson Aalen?

## When to use Nelson Aalen?

Nelson-Aalen analysis is used to analyze how a given population evolves with time. This technique is mostly applied to survival data and product quality data.

**What is empirical survival function?**

Empirical survival function When there is no censoring, the general formula is: ˆS(t)=# of individuals with T≥ttotal sample size. In most software packages, the survival function is evaluated just after time t, i.e., at t+. In this case, we only count the individuals with T>t.

**How do you calculate survival function?**

The survival function is S(t) = Pr(T >t)=1 − F(t). – The survival function gives the probability that a subject will survive past time t.

### How does a Kaplan Meier curve work?

The visual representation of this function is usually called the Kaplan-Meier curve, and it shows what the probability of an event (for example, survival) is at a certain time interval. If the sample size is large enough, the curve should approach the true survival function for the population under investigation.

**How do you describe survival curves?**

The horizontal axis (x-axis) represents time in days, and the vertical axis (y-axis) shows the probability of surviving or the proportion of people surviving. The lines represent survival curves of the two groups. A vertical drop in the curves indicates an event.

**How do you calculate a Kaplan Meier curve?**

With the Kaplan-Meier approach, the survival probability is computed using St+1 = St*((Nt+1-Dt+1)/Nt+1). Note that the calculations using the Kaplan-Meier approach are similar to those using the actuarial life table approach.

#### How is Kaplan Meier survival analysis calculated?

The Kaplan-Meier estimate is the simplest way of computing the survival over time in spite of all these difficulties associated with subjects or situations. For each time interval, survival probability is calculated as the number of subjects surviving divided by the number of patients at risk.

**What is the difference between Kaplan-Meier and Nelson-Aalen?**

While the Nelson-Aalen estimator is far less popular than the Kaplan-Meier survival curves, understanding it might be very helpful while working with more advanced methods of survival analysis, such as the Cox regression. You can find the code used for this article on my GitHub.

**What does the shape of the Nelson–Aalen estimator’s curve mean?**

While the image above represents the hazard rate (not the cumulative one!), the shape of the Nelson–Aalen estimator’s curve gives us an idea of how the hazard rate changes over time.

## Is there a similar formula to the Kaplan-Meier formula?

All these terms are naturally similar to the ones in the Kaplan-Meier estimator’s formula. The Nelson-Aalen estimator, or more generally visualizing the hazard function over time, is not a very popular approach to survival analysis. That is because — in comparison to the survival function — explanation of the curves is not so simple and intuitive.

**Is it possible to transform Kaplan-Meier to hazard function?**

Unfortunately, we cannot transform the Kaplan-Meier estimate of the survival function to the hazard function. However, we can use another non-parametric estimator of the cumulative hazard function — the Nelson-Aalen estimator. In short, it is used to estimate the cumulative number of expected events within a certain period of time.