Biostatistics

Step 1: Understanding the Concept:
The question asks for the definition or method of calculating the mean deviation. Mean deviation (or mean absolute deviation) is a measure of the average distance between each data point and the mean of the dataset.

Step 2: Analyzing the options:
- (A) Adding all the values of the given dataset. This gives the sum of the data, not a measure of deviation.
- (B) Dividing the sum of all the values... by the number of observations... This is the definition of the arithmetic mean.
- (C) Dividing the sum of all the deviations from the mean... by the number of observations... This is the definition of mean deviation. It's important to note that "deviations" here implies the absolute values of the deviations , because the sum of the simple deviations is always zero. Given the options, this is the intended correct answer. The formula is: MD = .
- (D) Dividing the sum of squares of all the deviations from mean... by the number of observations... This is the definition of the population variance ( ).

Step 3: Final Answer:
Option (C) provides the correct description for calculating the mean deviation.

Step 1: Understanding the Concept:
The harmonic mean is a type of average, calculated as the reciprocal of the arithmetic mean of the reciprocals of the observations. It is particularly useful for averaging rates.

Step 2: Key Formula or Approach:
The formula for the Harmonic Mean (HM) is: where is the number of observations and are the individual values.

Step 3: Detailed Calculation:
1. Identify the data and n: The data points are 10, 20, 40, 60, 120. The number of observations, , is 5.
2. Calculate the reciprocals of each value:

3. Sum the reciprocals ( ):
To add these fractions, we find a common denominator, which is 120.
Simplifying the fraction: or 0.2.
4. Calculate the Harmonic Mean:

Step 4: Final Answer:
The Harmonic Mean of the given data is 25. Therefore, option (A) is correct.