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Lecture notes of 3 pages for the course STAT101 at UManitoba
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Confidence intervals are a powerful tool for data analysis that allows us to estimate unknownpopulation parameters from sample data. In this lecture, we will define what a confidence
interval is and how it is calculated, discuss the concept of point estimates and interval
estimates, explore different types of confidence intervals, and learn how to interpret and
misinterpret confidence intervals.
1. Definition of Confidence Interval:
A confidence interval is a range of plausible values for an unknown population parameter,
calculated from sample statistics. The confidence level associated with a confidence interval
represents the percentage of times that the interval would contain the true population parameter
if we were to repeat the sampling process many times. Typically, a confidence level of 95% is
used, meaning that for 95% of all samples, the corresponding confidence interval will contain
the desired population parameter.
2. From Point Estimates to Interval Estimates:
Point estimates are single values that are used to estimate an unknown population parameter
from sample data. Examples of point estimates include the sample mean, sample variance, and
the sample proportion. However, point estimates can be imprecise and may not accurately
reflect the true population parameter. Interval estimates, on the other hand, provide a range of
plausible values for the unknown population parameter, based on sample data. Confidence
intervals are a type of interval estimate.
3. Confidence Intervals and Margins of Error:
The margin of error is the amount of error that is associated with a particular confidence interval.
It represents the maximum distance that the sample statistic (e.g., sample mean) is expected to
deviate from the true population parameter. The margin of error is influenced by the sample
size, confidence level, and variability of the population. The larger the sample size, the smaller
the margin of error. Similarly, increasing the confidence level or reducing the variability of the
population will also result in a smaller margin of error.
4. Confidence Intervals: Examples
Several types of confidence intervals can be used depending on the type of data being analyzed
and the research question being asked. Some examples include:
● Confidence interval for a population mean: This type of interval estimate is used to
estimate the true population means when the population standard deviation is known.
The formula for this confidence interval is:
̄X ± Z(α/2) (σ/√n)
Where X̄ is the sample mean, Z(α/2) is the z-score corresponding to the desired confidence
level, σ is the population standard deviation, and n is the sample size.
● Confidence interval for a population mean with unknown standard deviation: This type of
interval estimate is used to estimate the true population means when the population
standard deviation is unknown. The formula for this confidence interval is:
̄X ± t(α/2, n-1) (s/√n)
Where X̄ is the sample mean, t(α/2, n-1) is the t-score corresponding to the desired confidence
level and degrees of freedom, s is the sample standard deviation, and n is the sample size.
● Confidence interval for a population proportion: This type of interval estimate is used to
estimate the true population proportion when data is categorical. The formula for this
confidence interval is:
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