Sentence:  Following observation of the fact that the reaction rates of supposedly identical reaction mixtures prepared on the same filling manifold and exposed under identical conditions <WSD>often</WSD> differed by several hundred per cent , a systematic series of experiments was undertaken to see whether the difficulty could be ascribed to the method of preparing the chlorine , to the effects of oxygen or moisture or to the effect of surface to volume ratio in the reaction tubes . 
Meaning of the word marked with <WSD> tags:  many times at short intervals