ISEB ISEB-PM1 Exam Given the following values, what is the calculated expected activity cost (CE) when using PERT? CO = $7,000 CM = $10,500 CP = $11,000

Question

Options

• A$ 4,750
• B$ 9,500
• C$10,000
• D$11,500

• A
  4% (1)
• B
  12% (3)
• C
  77% (20)
• D
  8% (2)

Explanation

Option C ($10,000) is correct because the PERT expected cost formula is CE = (CO + 4×CM + CP) ÷ 6, which weights the most likely estimate four times more heavily than the optimistic or pessimistic estimates. Plugging in the values: (7,000 + 4×10,500 + 11,000) ÷ 6 = 60,000 ÷ 6 = $10,000.

Why the distractors are wrong:

• B ($9,500) is the simple arithmetic mean - (7,000 + 10,500 + 11,000) ÷ 3 - which ignores PERT's critical 4× weighting of the most likely estimate; this is the most tempting trap.
• A ($4,750) and D ($11,500) result from incorrect formula structures (e.g., wrong denominators or omitting the 4× multiplier) and don't correspond to any valid PERT calculation.

Memory tip: Think of PERT as the "1-4-1 rule" - Optimistic gets weight 1, Most Likely gets weight 4, Pessimistic gets weight 1, and you divide by their sum (6). The heavy middle weighting reflects that reality usually lands near the most likely estimate.

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