[英]Implied volatility calculator is wrong
我是一名計算機科學家,正在嘗試更多地了解量化金融。 我有一個用於計算 Black-Scholes 模型中歐式看漲期權價值的程序,我正在嘗試添加一種方法來計算隱含波動率。
import math
import numpy as np
import pdb
from scipy.stats import norm
class BlackScholes(object):
'''Class wrapper for methods.'''
def __init__(self, s, k, t, r, sigma):
'''Initialize a model with the given parameters.
@param s: initial stock price
@param k: strike price
@param t: time to maturity (in years)
@param r: Constant, riskless short rate (1 equals 100%)
@param sigma: Guess for volatility. (1 equals 100%)
'''
self.s = s
self.k = k
self.t = t
self.r = r
self.sigma = sigma
self.d = self.factors()
def euro_call(self):
''' Calculate the value of a European call option
using Black-Scholes. No dividends.
@return: The value for an option with the given parameters.'''
return norm.cdf(self.d[0]) * self.s - (norm.cdf(self.d[1]) * self.k *
np.exp(-self.r * self.t))
def factors(self):
'''
Calculates the d1 and d2 factors used in a large
number of Black Scholes equations.
'''
d1 = (1.0 / (self.sigma * np.sqrt(self.t)) * (math.log(self.s / self.k)
+ (self.r + self.sigma ** 2 / 2) * self.t))
d2 = (1.0 / (self.sigma * np.sqrt(self.t)) * (math.log(self.s / self.k)
+ (self.r - self.sigma ** 2 / 2) * self.t))
if math.isnan(d1):
pdb.set_trace()
assert(not math.isnan(d1))
assert(not math.isnan(d2))
return (d1, d2)
def imp_vol(self, C0):
''' Calculate the implied volatility of a call option,
where sigma is interpretered as a best guess.
Updates sigma as a side effect.
@rtype: float
@return: Implied volatility.'''
for i in range(128):
self.sigma -= (self.euro_call() - C0) / self.vega()
assert(self.sigma != -float("inf"))
assert(self.sigma != float("inf"))
self.d = self.factors()
print(C0,
BlackScholes(self.s, self.k, self.t, self.r, self.sigma).euro_call())
return self.sigma
def vega(self):
''' Returns vega, which is the derivative of the
option value with respect to the asset's volatility.
It is the same for both calls and puts.
@rtype: float
@return: vega'''
v = self.s * norm.pdf(self.d[0]) * np.sqrt(self.t)
assert(not math.isnan(v))
return v
這是我目前擁有的兩個測試用例:
print(BlackScholes(17.6639, 1.0, 1.0, .01, 2.0).imp_vol(16.85))
print(BlackScholes(17.6639, 1.0, .049, .01, 2.0).imp_vol(16.85))
頂部的打印出 1.94,這與http://www.option-price.com/implied-volatility.php給出的 195.21% 的值相當接近。 但是,底部的(如果刪除斷言語句)會打印出 'nan' 和以下警告消息。 使用 assert 語句, self.vega()
在 imp_vol 方法中返回零,然后assert(self.sigma != -float("inf"))
。
so.py:51: RuntimeWarning: divide by zero encountered in double_scalars
self.sigma -= (self.euro_call() - C0) / self.vega()
so.py:37: RuntimeWarning: invalid value encountered in double_scalars
+ (self.r + self.sigma ** 2 / 2) * self.t))
so.py:39: RuntimeWarning: invalid value encountered in double_scalars
+ (self.r - self.sigma ** 2 / 2) * self.t))
你正在做的事情沒有多大意義。 您正試圖收回大量資金、短期期權的隱含波動率。 此選項上的 vega 將有效地為 0,因此您獲得的隱含 vol 數將毫無意義。 浮點舍入給了你無限的體積,我一點也不感到驚訝。
如果您使用 vega 來估計隱含波動率,您可能正在執行牛頓梯度搜索的一些變體,它不會在所有情況下收斂到解決方案,我在 R 或 VBA 中編程,因此只能為您提供一個解決方案來翻譯,二分搜索方法簡單穩健且始終收斂,來自撰寫完整的期權定價模型書籍的人是 Espen Haugs 算法,用於二分搜索以找到隱含波動率;
在搜索隱含波動率時,Newton-Raphson 方法需要了解期權定價公式相對於波動率 (vega) 的偏導數。 對於某些期權(特別是異國期權和美式期權),vega 是未知的。 當 vega 未知時,二分法是一種更簡單的估計隱含波動率的方法。 二分法需要兩個初始波動率估計值(種子值):
Function GBlackScholesImpVolBisection(CallPutFlag
As String, S As Double,
X As Double, T As Double, r As Double, _
b As Double, cm As Double) As Variant
Dim vLow As Double, vHigh As Double, vi As Double
Dim cLow As Double, cHigh As Double, epsilon As
Double
Dim counter As Integer
vLow = 0.005
vHigh = 4
epsilon = le-08
cLow = GBlackScholes ( CallPutFlag , S, X, T, r, b, vLow)
cHigh = GBlackScholes ( CallPutFlag , S, X, T, r, b, vHigh)
counter = 0
vi = vLow + (cm — cLow ) * (vHigh — vLow) / ( cHigh — cLow)
While Abs(cm — GBlackScholes ( CallPutFlag , S, X, T, r, b, vi )) > epsilon
counter = counter + 1
If counter = 100 Then
GBlackScholesImpVolBisection
Exit Function
End If
If GBlackScholes ( CallPutFlag , S, X, T, r, b, vi ) < cm Then
vLow = vi
Else
vHigh = vi
End If
cLow = GBlackScholes ( CallPutFlag , S, X, T, r, b, vLow)
cHigh = GBlackScholes ( CallPutFlag , S, X, T, r, b, vHigh )
vi = vLow + (cm — cLow ) * (vHigh — vLow) / ( cHigh — cLow)
Wend
GBlackScholesImpVolBisection = vi
End Function```
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