Balance scales, like those at doctor's offices and gyms, can be more accurate than home, spring-loaded scales, if they are periodically re-calibrated. That's what the big weight at the left of the balance is for. To perform a quick check, set the small weights to zero, and see if the balance indicator at the right of the scale goes to the middle (or bobs up and down the same distance from the middle).

Even when properly calibrated, one's weight can be different on these scales compared a spring-loaded scale. As others have mentioned, clothing (generally about 2 lbs for clothing + shoes), time of day, hydration levels, and for women, menstrual periods, affect the result. So, pick a consistent time of day and liquid consumption amount for your weigh-ins. Women will have to factor in fluid retention during their menstrual cycles.

My personal experience, as a man, is that my fancy, new, spring-loaded Tanita home scale (which purports to measure body fat), is quite accurate in regard to my weight. It corresponded well to the scale the last time I went to the doctor's office a month ago, and to the electronic scale my trainer used at the gym.