Photosynthetic efficiency is often quantified as the light-limited, maximum quantum yield in ecophysiological studies. Four published comparative studies report that photosynthetic efficiency varies little among plant species of widely diverse origins, and that quantum yields were near the maximum theoretically attainable value. However, many other published studies contradict this conclusion, reporting quantum yields as low as 30% of those found in the comparative studies. These studies have created the impression that certain plants, particularly wild plants growing outdoors, may have intrinsically low photosynthetic efficiencies. To investigate the validity of these differing interpretations, we compiled quantum yield data from a survey of 30 published studies and compared those with data from the two most comprehensive comparative quantum yield studies. We also included quantum yield observations that we made on ten species. While our data confirm the results of the comparative studies indicating that maximum quantum yield is high and invariant, the literature survey data showed a wide range of quantum yield values. To investigate whether low quantum yield values could be caused by data collection and analysis techniques, we analyzed photosynthetic light-response data. Substantial underestimation of quantum yield could result from including in the calculation data extending beyond the linear region of the photosynthetic light response. In some cases quantum yield measurements can be influenced by changing levels of intercellular CO2 during measurements. We conclude that many quantum yield values reported in the literature are affected by one or more of these errors, and the intrinsic efficiency of photosynthesis is mostly invariant among C3 plants. This emphasizes the importance of the measurement and data analysis protocols in obtaining accurate and reliable quantum yield data.