Answer: 0.23%

The standard deviation of the sample mean (also called standard error of the mean) is calculated using the formula: $$ \text{Standard Error} = \frac{\sigma}{\sqrt{n}} $$ Where: - σ = population standard deviation = 1.8% - n = number of observations Since we have monthly data for 5 years: - Number of months = 5 years × 12 months/year = 60 months Calculation: $$ \text{Standard Error} = \frac{1.8\%}{\sqrt{60}} = \frac{1.8\%}{7.746} = 0.232\% \approx 0.23\% $$ This represents the standard deviation of the sample mean, which measures how much the sample mean is expected to vary from the true population mean. The correct answer is 0.23%.

Author: Nikitesh Somanthe