DMfirst / Journal
Probability worked examplesDMfirst-owned · Edition 109

Calculate streak probabilities without predicting the next result

Work through a fair-coin model to distinguish the chance of a whole sequence from the chance of its next event. The arithmetic illustrates independence; it does not certify the behavior of any real game.

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Inside this edition

One topic. 6 useful perspectives.

6 sections

01

State the model before calculating

Probability calculations depend on assumptions. For this guide, assume a fair coin with independent tosses: each toss has a one-half chance of heads, and knowing earlier results does not change that chance. This is a mathematical model, not a statement about an online game's mechanism. A real activity may have different outcomes, probabilities, rules or dependencies that need separate evidence.

State the model before calculating
02

Calculate a specified sequence

Under the model, the probability of three heads in a row is one-half multiplied by one-half multiplied by one-half, or one-eighth. For ten specified heads, it is one divided by 1,024. Multiplication is appropriate because the events are independent and the sequence is specified in advance. The result describes the chance of that entire run occurring in those particular positions.

Calculate a specified sequence
03

Separate the next event from the full run

Suppose nine heads have already occurred. Under the independent fair-coin model, the probability that the next toss is heads remains one-half. The probability of ten heads from the start was small, but the first nine results are now known. You are asking a new question about the remaining event, not rerunning the original calculation from an empty history.

Separate the next event from the full run
04

Notice when you changed the question

The chance of heads on the next toss, the chance of ten heads starting now, and the chance of at least one long run somewhere in many tosses are different quantities. A large collection of observations offers many opportunities for a striking pattern to appear. Looking through thousands of sequences and highlighting the most unusual one is not equivalent to predicting that sequence before observing the data.

Notice when you changed the question
05

Distinguish independence from mutual exclusivity

Two events are mutually exclusive if they cannot happen together in the situation being described. Heads and tails on the same single toss are mutually exclusive. Heads on the first toss and heads on the second toss are different events that can both occur and are independent under this model. OpenStax emphasizes that these terms describe different relationships.

Distinguish independence from mutual exclusivity
06

Use the example to read claims critically

When someone claims that a streak proves the next result, ask which assumptions connect the history to that prediction. Have the probabilities actually changed, or is the claim based only on a feeling that a pattern must reverse? A chart, confident explanation or exact-looking percentage does not supply missing evidence. A useful analysis separates the model, the calculation and the observations used to justify the model.

Use the example to read claims critically

Make the reading useful.

Take a moment to check your understanding. This is a reflection aid, not a certification or account check.

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