Applying machine learning to real problems is non-trivial because many important steps are needed to prepare for learning and to interpret the results after learning. This dissertation investigates four problems that arise before and after applying learning algorithms. First, how can we verify a dataset contains "good" information? I propose cross-data validation for quantifying the quality of a dataset relative to a benchmark dataset and define a data efficiency ratio that measures how efficiently the dataset in question collects information (relative to the benchmark). Using these methods I demonstrate the quality of bird observations collected by the eBird citizen science project which has few quality controls. Second, can off-the-shelf algorithms learn a model with good task-specific performance, or must the user have expertise both in the domain and in machine learning? In many applications, standard performance metrics are inappropriate, and most analysts lack the expertise or time to customize algorithms to optimize task-specific metrics. Ensemble selection offers a potential solution: build an ensemble to optimize the desired metric. I evaluate ensemble selection's ability to optimize for domain-specific metrics on natural language processing tasks and show that ensemble selection usually improves performance but sometimes overfits. Third, how can we understand complex models? Understanding a model often is as important its accuracy. I propose and evaluate statistics for measuring the importance of inputs used by a decision tree ensemble. The statistics agree with sensitivity analysis and, in an application to bird distribution models, are 500 times faster to compute. The statistics have been used to study hundreds of bird distribution models. Fourth, how should data be pre-processed when learning a high-performing ensemble? I examine the behavior of variable selection and bagging using a bias-variance analysis of error. The results show that the most accurate variable subset corresponds to the best bias-variance trade-off point. Often, this is not the point separating relevant from irrelevant inputs. Variable selection should be viewed as a variance reduction method and thus is often redundant for low variance methods like bagging. The best bagged model performance usually is obtained using all available inputs.