struct Observation {
    var podcastState: PodcastState
    var userSpeed: Float
}

func meanError(for model: PodcastSpeedModel, _ observations: [Observation]) -> Float {
    var error: Float = 0
    for observation in observations {
        error += abs(model.prediction(for: observation.state) - observation.userSpeed)
    }
    return error / Float(observations.count)
}