def throughput_history(summaries):
    """Calculates the change in completion for a list of summaries.
    Args:
        summaries (List[ProjectSummary]): List of project summaries to analyze
    Returns:
        List[int]: The change in number of items complete between each set of two summaries provided.
    """
    history = []
    for i in range(len(summaries)-1):
        throughput = summaries[i+1].complete - summaries[i].complete
        if throughput < 0:
            throughput = 0
        history.append(throughput)
    return history