How many records
Ten independent values are drawn from the same continuous distribution and observed in this order. is a record if it is larger than all of (the first value is always a record).
Find the expected number of records.
Fractions, decimals with at least 3 significant digits, and expressions such as sqrt(2)/2, e^-1 or pi/4.
Write the number of records as a sum of indicators .
is a record when it is the largest of the first values. Each of those values is equally likely to be the largest.
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s Shown after a correct answer. Revealing it first marks the problem as seen, not solved.
Idea. Linearity of expectation turns the count into a sum of probabilities, and each probability follows from symmetry among the first values.
Step 1: indicators. The number of records is , where if is a record.
Step 2: one indicator. is a record exactly when it is the maximum of . The values are i.i.d. and continuous (no ties), so each of the values is the maximum with the same probability: .
Step 3: add.
Remarks. The answer does not depend on the distribution. With values there are about records, so records become rare: in a million observations you expect only about . The indicators are in fact independent, though linearity did not need it.