how : string
			value
			pct_chg
			diff

def correl(df,periods=21,columns=None,include=True,str=None,detail=False,how='value',**correl_kwargs):
	"""
		how : string
			value
			pct_chg
			diff
	"""
	def _correl(df,periods=21,columns=None,include=True,str=None,detail=False,**correl_kwargs):
		study='CORREL'
		df,_df,columns=validate(df,columns)

		_df['CORREL'] = df[columns[0]].rolling(window=periods,**correl_kwargs).corr(df[columns[1]])

		str=str if str else 'CORREL({column1},{column2},{period})'.format(column1=columns[0],column2=columns[1],period=periods)
		return rename(df,_df,study,periods,columns,include,str,detail)
	columns=df.columns if not columns else columns
	if len(columns) != 2: 
		raise StudyError("2 Columns need to be specified for a correlation study")
	periods=make_list(periods)
	if how=='pct_chg':
		df=df[columns].pct_change()
	elif how=='diff':
		df=df[columns].diff()
	__df=pd.concat([_correl(df,columns=columns,periods=y,include=False,str=str) for y in periods],axis=1)
	if include:
		return pd.concat([df,__df],axis=1)
	else:
		return __df