By now you are adept at calculating averages and intuitively can estimate whether something is “normal” (a measurement not too far from average) or unusual (pretty far from the average you might expect). This class helps to quantify exactly how far something you measure is from average using the normal distribution. Basically, you mark the mean down the middle of the bell curve, calculate the standard deviation of your sample, and then add (or subtract) that value to come up with the mile markers (z-scores) that measure the distance from the mean.

For example, if the average height of adult males in the United States is 69 inches with a standard deviation of 3 inches, we could create the graph below.

Men who are somewhere between 63 and 75 inches tall would be considered of a fairly normal height. Men shorter than 63 inches or taller than 75 inches would be considered unusual (assuming our sample data represents the actual population). You could use a z-score to look up exactly what percentage of men are shorter than (or taller than) a particular height.

Think of something in your work or personal life that you measure regularly. (No actual calculation of the mean, standard deviation, or z-scores is necessary.) What value is “average”? What values would you consider to be unusually high or unusually low? If a value were unusually high or low—how would it change your response to the measurement?

SOLUTION

What Value Is “Average”?

For adults, an average or “normal” blood pressure is typically considered to be around 120/80 mmHg. This is the standard baseline used in both hospital and outpatient settings. We are trained to consider readings consistently close to this number as within normal limits, especially in otherwise healthy individuals.

What Values Are Unusually High or Low?

Unusually high: A systolic reading above 140 mmHg or a diastolic above 90 mmHg would be considered hypertensive, which may indicate that the patient is at increased risk for stroke or heart disease. In some cases—like a reading of 180/110 mmHg or higher—this is a hypertensive crisis, requiring immediate intervention.
Unusually low: A blood pressure reading below 90/60 mmHg is considered hypotensive and may suggest that the patient is experiencing dehydration, blood loss, or other critical issues.

How Would an Unusually High or Low Value Change My Response?

If I receive a high reading, I would:

Recheck the measurement manually to confirm accuracy.
Assess for symptoms (e.g., headache, chest pain, visual disturbances).
Notify the provider immediately if it’s critically elevated.
Review the patient’s medication and compliance, if applicable.

If the value is unusually low, I would:

Immediately assess for dizziness, fainting, or altered mental status.
Check for bleeding, sepsis, or signs of dehydration.
Place the patient in a supine position, start fluids if ordered, and notify the provider.

Z-Scores and Interpretation in Practice

While I don’t calculate z-scores at the bedside, the concept of deviation from the mean is something I instinctively consider when determining whether a patient’s vital signs are normal or worrisome. If a patient’s systolic pressure is 120, I know it’s typical. If it’s 160, I realize that it’s likely more than one standard deviation above the norm, which may represent a significant clinical issue.

Conclusion

Understanding what counts as “normal” or “unusual” helps guide how urgently we respond to a clinical measurement. Even though we don’t always do the math explicitly, concepts like mean, standard deviation, and the normal distribution are embedded in how healthcare professionals assess patient safety and intervene effectively.

patients’ blood pressure readings.