### CONFIDENCE INTERVAL CALCULATOR

Level of accuracy in percentages observed:

To conduct market research is to interview (or observe) a sample of individuals. The larger the sample, the greater the statistical accuracy. Confidence interval provides the level of accuracy of a given measure.

#### For instance

Upon interviewing a sample of 500 GP’s, 20% are found to be using Product A. So, what is, within the total population, the actual proportion of users of Product A.

Assuming 5% risk level*, confidence interval is:

• 20% +/-3.5

• [16.5%-23.5%]

In other words, there is a 95% probability that the actual percentage of Product A users in the total population of GPs is between 16.5% and 23.5%.

### COMPARING TWO PERCENTAGES

Statistical significance between 2 observed percentages:

When comparing opinions (or attitudes) measured on two samples, a significance test is applied. Significance tests provide an indication whether observed percentages are significantly different or not.

#### For instance

Upon interviewing a sample of 500 GP’s, 20% are found to be using Product A. Equally, upon interviewing a sample of 150 cardiologists, 25% of those are found to be using Product A. Would such a result suggest that a larger proportion of cardiologists is using Product A?

At 5% risk level*, the two percentages observed on these samples are not statistically different.

In other words, there is a 95% probability that the actual proportion of Product A users among GPs’ total population is similar to the actual proportion of Product A users among cardiologists’ total population.

### SAMPLE SIZE CALCULATOR

Minimum sample size calculations:

By taking into account an expected percentage, as well as the corresponding confidence interval, it is possible to calculate the sample size needed for a given market research survey.

This application calculates the minimum sample of respondents needed in order to achieve the required level of research accuracy.

#### For instance

Research is needed to evaluate the level of use of Product A among anaesthetists. The desired confidence interval is +/-8. Approximately one third of anaesthetists is expected to be using Product A.

How many anaesthetists need to be interviewed in order to achieve this margin of error?

At 5% risk level*, for an expected percentage of 33%, with a confidence interval of +/-8, minimum sample size is 133 anesthetists.

In other words, if you interview a minimum of 133 anesthetists, there is a 95% chance to get a confidence interval of +/-8 if the proportion of Product A users in the anesthetist sample is 33%.