Hi, the bias can not be entirely avoided but it can be minimalised. The first thing is to make sure that the people, who do the measurements are properly trained, second that they use standardised and calibrated instruments and third that they measure according to the same protocol in as standardised environment as possible. Having one person measuring everybody can be a benefit but it may also be a risk. Namely, the same person probably produces similar systematic error, so the error is similar across the measurements. On the other hand, you have to take into consideration that overburdening one person with huge volume of measurements can lead to fatigue and when people are tired they tend to make more mistakes. I would suggest a well trained team of people instead of one person, if we are talking about more than 100 measurements per day. We usually measure up to 300 per day and have 2 people per two measurements, e.g. weight and height is measured by a team of two who measure interchangeably. If you are measuring height, make sure you do it in the morning hours and if you are measuring weight, make sure you do it before meals.

You need to make sure that each one knows how to measure (calibration against a gold standard/examiner training), you need to make sure that all the trainers do their measurement in the same way (inter examiner reliability), and lastly that they are taking their measurements in consistent manner (intra examiner reliability/test retest reproducibility). And you should analyse their agreement statistically using a Kappa test, and results that are good and above reflect good internal validity. If the kappa result is low, you will need to train again. All this should be completed prior to starting your data collection.