def significance(p):
    """
    Returns significance symbol based on set alpha values
    :param p: probability of statistical test
    :return: string expression for significance
    """
    if p < 0.001:
        expression = "***"
    elif p < 0.01:
        expression = "**"
    elif p < 0.05:
        expression = "*"
    else:
        expression = "ns"
    return expression