Measurement interruptions can be easily identified on the 1-hour window graph. Occasional interruptions caused by clock chimes are acceptable and become less important the longer the measurement. However, if you notice lots of missing spots, the results are likely skewed because of poor signal pickup and/or excessive background noise. In such cases, repeat the setup and provide a quieter environment.
When the pendulum is started, it may need time to settle and stabilize. This can take 15 minutes or more. During this time a big increase or decrease in beat rate could occur. Watch the plot and consider restarting the measurement after the pendulum has settled. This will ensure that the reported average will only reflect the stable data.
Always enter the target rate up to at least the tenths, for example 5642.45 instead of 5642. Avoid rounding as it decreases precision. Depending on the beat rate number, the tenths could account for loss of precision up to ±12 S/D, and the hundreds of ±1.2 S/D.
Note that published beat rate data may not always be accurate. Learn more
Mechanical clock speed is not constant. The build quality of the movement, age and wear, spring tension and even temperature can all cause fluctuations. That's why the ClockMaster app was specifically designed for long-term measurements.
A couple of hours measurement reveal patterns specific to the clock under test. On the 'instantaneous' green plot, these patterns can be observed, some caused by wheels rotation, some by temperature swings, and by the effect of the spring tension unequal power release for the case of spring-driven clocks.
The data can be exported for further analysis in a spreadsheet.
After a test is complete, the Export 
button is available to save the results to the iOS Files app. You can choose a file name and enter two lines of info text, such as clock make and model, owner info, etc.
For each export, two separate files are created with .pdf and .csv extensions.
The PDF report resembles the app Chart Display and is designed to be printed, stored with the clock, or sent to clients. It can be customized with your own business name and contact information (app settings tab).
The .csv file contains the raw data sampled every minute and can be imported to your preferred spreadsheet for detailed analysis.
ClockMaster app uses the iPhone audio ADC as the timing source. The ADC quartz oscillator frequency may vary from one device to another, within very small tolerances, resulting in error difference less than 0.9 S/D.
This is usually neglectable, however if you have access to a reference timing source such as GPS Disciplined Oscillator (GPSDO), it would be possible to compensate for this difference.
Measure the PPS output of the GPSDO and note the Error rate. Adjust the Error Correction app setting accordingly. For example if the measured GPSDO error was -0.4 S/D, the correction should be set to +0.4 S/D. Measure again, and you should get exactly 0.0 S/D.
Updated March 2020: To deal with variety of situations, many assumptions are made about the length of tick intervals, the beat variance and so on. The ClockMaster app only locks to beat when all the different checks pass. In extreme cases, with movements very inconsistent from one tick to the other, the app may fail to lock to beat at all. The experimental "BEAT LOCK: LOOSE" setting can be used to disable some of the checks and ignore the variance inconsistency. This comes with the increased risk of the slightest room noise to interfere with the measurement. So extreme silence is a must when this option is selected.