Statistics and Probability

Averages from a frequency table, comparing two sets of data honestly, and the difference between the probability you calculate and the one you observe.

หลักสูตรแกนกลาง / สสวท.: สถิติและความน่าจะเป็น (Statistics and Probability)ตรงกับ ม.2 บทที่ 7 สถิติ และต่อไปยังความน่าจะเป็นใน ม.3
Common Core (US): 7.SP, 8.SP
Cambridge: Lower Secondary Stage 8 — Statistics and Probability

11.1 Averages from a frequency table

The mean from a frequency table

1Multiply each value by its frequency
2Add those products to get the total
3Add the frequencies to get how many items there are
4Divide the total by the number of items

ตัวอย่าง

In a class, $5$ students scored $6$ , $8$ scored $7$ , $4$ scored $8$ and $3$ scored $9$ . Find the mean.

วิธีทำ

1.Products: $30$ , $56$ , $32$ , $27$
2.Total of scores: $145$
3.Number of students: $5 + 8 + 4 + 3 = 20$
4. $145 \div 20$

ตอบ $7.25$

ข้อควรระวัง

Divide by the total FREQUENCY, not by the number of rows in the table. Dividing $145$ by $4$ gives $36.25$ , which is not even a possible score — a sanity check that catches it instantly.

เคล็ดลับ

The mean of a set of whole numbers need not be a whole number, and $7.25$ is a perfectly good answer. Rounding it to $7$ because scores are whole throws away the information the mean was for.

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