Final answer:
For a variable measured on an interval scale with a normal distribution, the mean should be computed to find the average as the mean equals the median and mode in a normal distribution.
Step-by-step explanation:
If your variable is measured on an interval scale and is normally distributed, you should compute mean to find the average. This is because in a normal distribution, the mean, median, and mode all coincide at the same central point of the distribution. Since the question specifies that the distribution is normal, we can take advantage of this property.
To further clarify, in a symmetrical distribution, which includes the normal distribution, the relationship among the mean, median, and mode is that they all have the same value. Consequently, any of these measures could describe the center of the distribution but the mean is most commonly used, especially in a perfectly normal distribution.
The standard deviation is a measure of the spread of the data and not a measure of central tendency. Therefore, although it is important for understanding the variability within a dataset, it does not represent an average value.