This article describes the confidence interval as it is reported in the results of a Pulsewave® reading. It also describes why the confidence interval can vary from reading to reading. Confidence interval is shown in the results because indirect blood pressure measurement is not exact.
When viewing results in the Pulsewave® Software or on the Pulsewave® website, one may notice that there is a ± value reported next to the data. This value is known as the confidence interval (CI). The CI is the interval where the result is expected to be. Another way of looking at this is that the CI indicates the reliability of the reading to produce a result with the least amount of variability.
Confidence interval can be illustrated through an analogy to weather forecasting. Based on readings of various weather conditions at a particular time, forecasters can report their expectation of an amount of rainfall. The more reliable the readings, the less variability will be shown in the report. For example, if the report indicates that there will be between 10 and 50mm of rain on a given day, it is expressing that there will be 30mm of rain ±20mm. After taking a more reliable reading, the forecaster’s report could indicate that there will be between 25 and 35mm of rain. This means that there could be 30mm of rain but with a smaller variability of ±5mm. Since the reliability of the readings increased, the variability of the amount of rainfall decreased.
CI as shown in the Summary and Details view of a reading on the Pulsewave® website
The CI is calculated for one reading at one time, only. This value can change from reading to reading, but is not related between readings. The value is thus dependent on the reliability of one specific reading. With the Pulsewave® reading, a lower CI means that the interval within which the result is expected to be is smaller, indicating less variability in the result. For example, 122 ±2 mmHg means that the result can reasonably be between 120 and 124 mmHg. Conversely, a larger CI means that the interval within which the result is expected to be is larger, indicating more variability in the result. For example, 122 ±8 mmHg means that the result can reasonably be between 114 and 130 mmHg.
It must be noted that CI is not a measurement of a degree of error, nor is it a reflection of the accuracy of the result. It is a measurement of uncertainty. For further information about the topic of uncertainty, refer to Additional Resources, below.
What can impact CI?
Many different factors can impact the quality of the reading, in turn determining the CI. By following some simple guidelines with regards to external influences, you can ensure that you are submitting the best reading possible. The following are some things that are within your control to keep in check while taking a reading:
- Do not talk or move
- Eliminate vibrations on the desk or floor
- Sit comfortably and try to be relaxed
Uncertainty of Measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995), ISO/IEC Guide 98-3, 2008.