def denormalize(splits, norm):
    """
    Denormalize training, validation and test dataframe.
    Args:
        splits: Dictionary with training, validation and test normalized dataframe.
        norm: DataFrame with the parameters used in normalization process.

    Returns: Dictionary with training, validation and test dataframe before normalization.

    """
    mean = norm['mean']
    std = norm['std']

    for i in ['train', 'val', 'test']:
        splits[i]['X'] = splits[i]['X'] * std + mean

    return splits