Mga plan

Z-Score Calculator (may Percentile)

Hinahanap ng calculator na ito kung ilang standard deviation ang layo ng isang halaga sa mean, z = (x − μ)/σ. Ibinibigay rin nito ang porsiyento ng normal distribution sa ibaba at sa itaas ng halaga, para magawa mong percentile ang z-score.

Resulta

Ilagay ang mga halaga at pindutin ang “Kuwentahin” para makita ang resulta.

Formula

z=x−μσz = \frac{x - \mu}{\sigma}

Subtract the mean, then divide by the standard deviation.

x=μ+zσx = \mu + z\sigma

Rearranged, to find the value for a given z-score.

Paano gamitin ang calculator na ito

  1. Enter the value, the mean and the standard deviation (which must be positive).
  2. Press Calculate to see the z-score and the percentages below and above it.

Mga halimbawang may solusyon

A test score of 85 when the mean is 70 and σ = 15

85 − 70 = 15, and 15 ÷ 15 = 1, so z = 1.

The score is 1 standard deviation above the mean: about 84.13% of a normal distribution is below it.

A value below the mean: x = 62, μ = 70, σ = 4

62 − 70 = −8, and −8 ÷ 4 = −2.

z = −2: only about 2.28% of values are lower.

z = 1.5: x = 130, μ = 100, σ = 20

(130 − 100) ÷ 20 = 1.5.

About 93.32% of values are below 130.

Mga karaniwang pagkakamali

  • Dividing by the variance instead of the standard deviation.
  • Subtracting the wrong way round, which flips the sign of z.
  • Using the percentages for data that is far from normally distributed.

Mga madalas itanong

What does a z-score tell you?

How many standard deviations a value is above (positive) or below (negative) the mean. z = 0 is exactly the mean; z = 2 is unusually high.

How do you turn a z-score into a percentile?

Use the normal distribution (a z-table): the percentage below z is the percentile. z = 1 is about the 84th percentile.

What is a good z-score?

It depends on the context. About 68% of normal data has z between −1 and 1, and about 95% between −2 and 2.