How to find Z-score (biostatistics)

The Z-score serves several purposes in statistics: Standardization: It allows for the comparison of scores from different distributions or datasets by standardizing the values. This is particularly useful when dealing with data from different sources or scales. Understanding the Position of a Value: A Z-score indicates how far and in what direction a value deviates from the mean. For example, a Z-score of 2 means the value is two standard deviations above the mean, while a Z-score of -1.5 means the value is one and a half standard deviations below the mean. Probability and Statistical Significance: In a normal distribution, Z-scores can be used to calculate the probability of a value occurring under the curve. They are also used in hypothesis testing to determine the statistical significance of a result. Identification of Outliers: Extreme Z-scores (typically those greater than 3 or less than -3) can indicate that a value is an outlier, or significantly different from the rest of the data. Biostatistical Problem: Suppose you are a researcher studying the systolic blood pressure levels of adults in a particular population. From previous studies, you know that the systolic blood pressure levels in this population are normally distributed. The mean systolic blood pressure is 120 mmHg, and the standard deviation is 15 mmHg. You are interested in assessing how an individual with a systolic blood pressure of 135 mmHg compares to the rest of the population. Solution: To determine this, we calculate the Z-score for the individual’s systolic blood pressure. Given: Individual’s systolic blood pressure, X=135 mmHg Mean systolic blood pressure, μ=120 mmHg Standard deviation, σ=15 mmHg We use the Z-score formula: Z= (X−μ)/SD Substitute the values: Z= (135−120)/15 ​ Now, let’s calculate the Z-score. The Z-score for the individual with a systolic blood pressure of 135 mmHg is 1.0. This means that the individual’s blood pressure is 1 standard deviation above the mean systolic blood pressure of the population. Interpreting the Z-score in this biostatistical context: A Z-score of 1.0 indicates that the individual’s systolic blood pressure is higher than the average for the population. The positive Z-score suggests that the individual’s blood pressure is above the population mean. While this individual’s blood pressure is higher than average, it’s only one standard deviation above the mean, which is typically considered within the range of normal variation in a population. However, it’s important to note that medical interpretation of blood pressure values also considers other factors, such as age, overall health, and medical history. ​