1. What is Variability?

Variability tells us how spread out the data is.

Even if two patients have the same average blood pressure, the fluctuations in their readings might be very different.

Average BP	Standard Deviation	Interpretation
130 mmHg	2 mmHg	Very stable
130 mmHg	15 mmHg	Fluctuating a lot

2. Key Measures of Variability

Measure	What It Tells You	How to Use It
Range	Difference between highest and lowest value	Simple spread
Interquartile Range (IQR)	Range of the middle 50% of data	Better for skewed data
Standard Deviation (SD)	How far data typically falls from the mean	Best when data is normally distributed

Values:120, 122, 125, 130, 135, 145

The SD tells us:Most patients have systolic BP within 8.4 mmHg of the average.

Scenario	Why Variability Matters
Glucose control	A stable patient has low variability in glucose readings
ICU vitals	High variability may signal instability or worsening
Lab results across patients	Tells us if most patients responded similarly to treatment
Blood pressure logs	Averages can be misleading without knowing variation

Imagine two diabetic patients with average glucose = 140 mg/dL:

Same mean, very different stability→ The second patient may need closer monitoring or medication adjustment.

Data Type	Use This Variability Measure
Symmetric, normal data	Standard Deviation
Skewed data or outliers	IQR
Quick overview	Range