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The Garmin HRM4 uses accelerometers in the chest strap to measure [[Running Dynamics]], including cadence. On the graph below, points falling on the green diagonal line accurate, having the same value on both devices. The further the points are from the green line less accurate they are. There is a blue regression line which in an accurate device would be in line with the green identity line. The points are added transparently to give an indication of density, with a full black color representing 100 overlapping readings. The chart represents well over 10,000 measurements, which I believe is enough to give a good indication of accuracy. The Garmin HRM4 has a trueness of -1.0 steps per minute (SPM), which suggests that over the course of a run, it will give you an average reading that is only one SPM too low. Its precision is 3.8 SPM, which means that at any given moment the reading is unlikely to be out by about four steps per minute, which is a little more than I would like.