95% Confidence Interval

Definition

A 95% confidence interval is a range of values, derived from sample data, that is likely to contain the true population parameter with a probability of 95%. This interval provides an estimate of the uncertainty around a sample statistic and implies that if we were to take many samples and compute a confidence interval for each one, approximately 95% of those intervals would contain the true population parameter. This concept plays a crucial role in evaluating claims and understanding the precision of sample estimates.

5 Must Know Facts For Your Next Test

1. The formula for calculating a 95% confidence interval for a population mean typically involves the sample mean, the critical value from the Z or t distribution, and the standard error.
2. In the context of comparing two population proportions, if the 95% confidence interval for their difference does not contain zero, it suggests a statistically significant difference between the populations.
3. A wider confidence interval indicates more uncertainty about the population parameter, while a narrower interval suggests greater precision based on the sample data.
4. The choice of a 95% confidence level is common in research but can be adjusted to 90% or 99% depending on the desired level of certainty.
5. Confidence intervals assume that the sampling distribution is approximately normal; thus, this method is most reliable when sample sizes are large or when using appropriate methods for small samples.

Review Questions

• How does a 95% confidence interval help justify claims about population proportions?
  • A 95% confidence interval can support claims about population proportions by indicating whether the estimated difference between two groups falls within a certain range. If this interval does not include zero, it suggests that there is a significant difference between the proportions of the two populations being compared. This helps researchers understand if their observed effects are likely due to chance or represent a true difference.
• Compare and contrast how to construct a 95% confidence interval for a population mean versus for a difference in population proportions.
  • Constructing a 95% confidence interval for a population mean involves calculating the sample mean and using either the Z or t distribution based on sample size. The interval is then formed by adding and subtracting the margin of error from the sample mean. In contrast, constructing an interval for the difference in population proportions requires determining each proportion from two separate samples, calculating their difference, and then applying formulas that account for their respective variances. The methods differ primarily in their calculations due to the nature of what is being estimated—mean versus proportion.
• Evaluate how changing the confidence level impacts the width of a confidence interval and interpret its implications for statistical conclusions.
  • Increasing the confidence level from 95% to 99% will result in a wider confidence interval because it allows for greater uncertainty in estimating the population parameter. This wider range means that while we gain more certainty that our interval includes the true parameter, we lose precision in our estimates. Conversely, lowering the confidence level to 90% creates a narrower interval but reduces our assurance that it captures the true parameter. Balancing between confidence levels and interval width is crucial as it directly affects how confidently we can make statistical conclusions based on our data.

Related terms

Margin of Error:

The amount of error that is allowed in the results of a survey or poll, calculated as part of the confidence interval.

Sample Mean:

The average value obtained from a sample, used to estimate the population mean.

Population Proportion:

The ratio of members with a particular characteristic to the total number of members in a population, often estimated using sample data.