Classroom observational protocols, in which raters observe and score the quality of teachers' instructional practices, are often used to evaluate teachers for consequential purposes despite evidence that scores from such protocols are frequently driven by factors, such as rater and temporal effects, that have little to do with teacher quality. In this study, we present an item response theory model that improves precision of observation scores by accounting for multiple sources of measurement error. Unlike the sum-scoring approach typically used in practice, the model presented here identifies raters and protocol items that are more reliable and should therefore be weighted more in scores. We then show that teachers identified as substandard using our model versus the sum-scoring approach are oftentimes very different. Our results suggest that accountability systems could improve observation scores by increasing the scoring weights of items and raters that provide greater information.