In Horn's Algorithm, the lower fence for outlier rejection is computed as:

• A. Q1 - 1.5 × IQR
• B. Q3 - 1.5 × IQR
• C. Q1 + 1.5 × IQR
• D. Q3 + 1.5 × IQR

Answer: (A)

The idea is to define a range that separates the central 50% of the data from potential outliers using the interquartile range (IQR). Outliers are values that fall beyond the fences set 1.5 times the IQR away from the quartiles. For the lower side, you move 1.5 IQRs below the first quartile (Q1). That gives the lower fence as Q1 − 1.5 × IQR. Any value smaller than that is considered a low outlier. The corresponding upper fence uses the third quartile plus 1.5 × IQR. So the correct expression for the lower fence is Q1 − 1.5 × IQR. For example, if Q1 is 10 and IQR is 4, the lower fence is 10 − 1.5×4 = 4; values below 4 are flagged as outliers.