# Breeden-Litzenberger formula for risk-neutral densities

Based on this topic: How to derive the implied probability distribution from B-S volatilities?

I am trying to implement the Breeden-Litzenberger formula to compute the market implied risk-neutral densities for the S&P 500 for some quoting dates. The steps that I take are as follows:

Step 1: Extract the call_strikes c_strikes for a given maturity T and the corresponding market prices css.

Step 2: Once I have the strikes and market prices, I compute the implied volatilities via the function ImplieVolatilities.m I'm 100% sure that this function works.

Step 3: Next, I interpolate the implied volatility curve by making use of the Matlab command interp1. Now, the vector xq is the strikes grid and the vector vqthe corresponding interpolated implied volatilities.

Step 4: Since I have to volatility curve, I can compute the market implied density: \begin{align} f(K) &= e^{rT} \frac{\partial^2 C(K,T)}{\partial K^2} \\ &\approx e^{rT} \frac{C(K+\Delta_K,T)-2C(K,T)+C(K-\Delta_K, T)}{(\Delta_K)^2} \end{align} where $\Delta_K = 0.2$ is the strike grid size. I only show the core part of the Matlab program:

c_strikes = Call_r_strikes(find(imp_vols > 0));
css = Call_r_prices(find(imp_vols > 0));

impvolss = ImpliedVolatilities(S,c_strikes,r,q,Time,0,css); %implied      volatilities
xq = (min(c_strikes):0.2:max(c_strikes)); %grid of strikes
vq = interp1(c_strikes,impvolss,xq); %interpolatd values %interpolated implied volatilities

f = zeros(1,length(xq)); %risk-neutral density
function [f] = secondDerivativeNonUniformMesh(x, y)
dx = diff(x); %grid size (uniform)
dxp = dx(2:end);
d2k = dxp.^2; %squared grid size
f = (1./d2k).*(y(1:end-2)-2*y(2:end-1)+y(3:end));
end

figure(2)
plot(xq,f);


Note: blsprice is an intern Matlab function so that one should definitely work. This is the risk-neutral pdf for the interior points:

The data The following table shows the option data (left column: strikes, middle: market prices and right column: implied volatilities):

                   650                     387.5                  0.337024
700                    346.45                  0.325662
750                     306.8                  0.313846
800                    268.95                  0.302428
850                    232.85                  0.290759
900                    199.15                  0.279979
950                     168.1                  0.270041
975                     153.7                  0.265506
1000                     139.9                  0.260885
1025                    126.15                  0.255063
1050                     112.9                  0.248928
1075                     100.7                  0.243514
1100                     89.45                  0.238585
1125                     79.25                  0.234326
1150                      69.7                  0.229944
1175                      60.8                   0.22543
1200                      52.8                  0.221322
1225                      45.8                   0.21791
1250                      39.6                   0.21486
1275                      33.9                   0.21154
1300                     28.85                  0.208372
1325                      24.4                  0.205328
1350                      20.6                  0.202698
1375                      17.3                  0.200198
1400                     13.35                  0.193611
1450                      9.25                  0.190194
1500                      6.55                  0.188604
1550                       4.2                  0.184105
1600                      2.65                  0.180255
1650                     1.675                  0.177356
1700                     1.125                  0.176491
1800                     0.575                  0.177828


$T = 1.6329$ is the time to maturity, $r = 0.009779$ the risk-free rate and $q = 0.02208$ the dividend yield. The spot price $S = 1036.2$.

Computation of the risk-free rate and dividend yield via Put-Call parity $r$ is the risk-free rate corresponding to maturity $T$ and $q$ is the dividend yield. In the numerical implementation, we derive $r$ and $q$ by again making use of the Put-Call parity. To that end, we assume the following linear relationship: $$f(K) = \alpha K-\beta,$$ where $\alpha = e^{-rT}$ and $-e^{-qT}S(0) = \beta$, and $f(K) = P(K,T)-C(K,T)$. The constants $\alpha$ and $\beta$ are then computed by carrying out a linear regression. Consequently, the risk-free rate $r$ and dividend yield $q$ are then given by \begin{align} r &= \frac{1}{T}\ln\left(\frac{1}{\alpha}\right), \nonumber \\ q &= \frac{1}{T} \ln \left(\frac{-S(0)}{\beta}\right). \nonumber \end{align}

• Could you please tell me where you got the data from. I am struggling to find market price on historical options, given a strike price Apr 14 '18 at 15:30

I assume that for approximating the second derivative of the call price $C (K,T)$ at the bounds of the strike domain (see first 2 "if" cases of the last for loop of your code) you tried to set up boundary conditions.

On the right bound, your approximation $C(K+\Delta K, T) \approx 0$ could make sense for $K \to \infty$ or at least big enough.

On the left bound however, $C(K-\Delta K, T)$ cannot be approximated by zero, since call price is almost only intrinsic value for low strikes. Instead of zero, you could use the discounted difference of the forward price minus the strike, $C(K-\Delta K, T) \approx B(0,T)(F (0,T)-K)$, if $K \to 0$ or at least small enough.

This explains why you observe a strange behaviour on the LHS (with a negative pdf), while I guess that if you zoom in or remove that outlier you should be OK.

As a first step, you could simply compute the pdf for interior points xq(2:end-1) (i.e. last case of your for loop) and check what it gives.

Looking more closely, it seems like your input prices allow for static arbitrage opportunities, more specifically butterfly arbitrage. As such you cannot expect to obtain a reasonable pdf.

Although it is far from perfect, maybe the script below will help. In it, I try to look for the strikes leading to arbitrage, and discard the corresponding inputs to obtain what a so-called "clean" pdf. This is a non-parametric approach. As discussed in the comments, using a parametric approach (e.g. fitting the vol smile using an arb free representation), will produce an even smoother pdf.

function [ ] = someBreedenLitzenbergerCode( )

% Get data and interpolate
[K, C, ~] = getData();
dk = 0.5;
k = min(K):dk:max(K);
c = interp1(K, C, k, 'spline');

% Notifying vertical arbitrage opportunities
dC = diff(C);
if any(dC > 0)
warning('Input call prices allow for arbtirage (vert. spread)');
end

% Notifying and dealing with butterfly arbitrage opportunities
f = secondDerivativeNonUniformMesh(k, c);
idxkArb = find(f<0)+1;
if ~isempty(idxkArb)
warning('Input call prices allow for arbitrage (butterfly)');
end

% Finding indices of input strikes/prices leading to arb
idxKArb = [];
for i = 1:length(idxkArb)
idxHi = find(K>=k(idxkArb(i)),1,'first');
idxLo = find(K<=k(idxkArb(i)),1,'last');
idxKArb = [idxKArb, idxHi, idxLo];
end
idxKArb = unique(idxKArb);
disp(K(idxKArb));

% Cleaning inputs and calculating pdf
K_clean = K; K_clean(idxKArb) = [];
C_clean = C; C_clean(idxKArb) = [];
k_clean = min(K_clean):dk:max(K_clean);
c_clean = interp1(K_clean, C_clean, k_clean, 'spline');
f_clean = secondDerivativeNonUniformMesh(k_clean, c_clean);

% Plotting results
if ishandle(1), clf(1); end;
figure(1);
subplot(221);
plot(K, C, 'x'); hold on;
plot(k, c, 'r-');
plot(k(idxkArb), c(idxkArb), 'rd');
grid on;
title('Original call prices, C(K)');
subplot(222);
plot(k(2:end-1), f); hold on;
plot(k(idxkArb),f(idxkArb-1), 'rd');
grid on;
title('Pdf, \phi(K)');
subplot(223);
plot(k_clean, c_clean); hold on;
lh.Box = 'off';
grid on;
title('Clean call prices, C(K)');
subplot(224);
plot(k_clean(2:end-1), f_clean); hold on;
a = ksr(k_clean(2:end-1), f_clean, 60); % Available at Matlab Central
plot(a.x, a.f, 'k-');
lh = legend('FD','FD + Smoothing');
lh.Box = 'Off';
grid on;
title('Pdf, \phi(K)');

end

function d2y_dx2 = secondDerivativeNonUniformMesh(x, y)

dx = diff(x);
dxp = dx(2:end);
dxm = dx(1:end-1);
d2k = dxp.*dxm;
d2kp = dxp.*(dxm+dxp);
d2km = dxm.*(dxm+dxp);
d2y_dx2 =  2./d2km .* y(1:end-2) - 2./d2k .*y(2:end-1) + 2./d2kp .*y(3:end);

end

function [strikes, prices, volatilities] = getData()

mat = [650        387.5      0.33702
700       346.45      0.32566
750        306.8      0.31385
800       268.95      0.30243
850       232.85      0.29076
900       199.15      0.27998
950        168.1      0.27004
975        153.7      0.26551
1000        139.9      0.26088
1025       126.15      0.25506
1050        112.9      0.24893
1075        100.7      0.24351
1100        89.45      0.23858
1125        79.25      0.23433
1150         69.7      0.22994
1175         60.8      0.22543
1200         52.8      0.22132
1225         45.8      0.21791
1250         39.6      0.21486
1275         33.9      0.21154
1300        28.85      0.20837
1325         24.4      0.20533
1350         20.6       0.2027
1375         17.3       0.2002
1400        13.35      0.19361
1450         9.25      0.19019
1500         6.55       0.1886
1550          4.2       0.1841
1600         2.65      0.18025
1650        1.675      0.17736
1700        1.125      0.17649
1800        0.575      0.17783];
strikes = mat(:,1);
prices = mat(:,2);
volatilities = mat(:,3);

end


• Thanks for the answer Quantuple. I have tried to plot the density for the interior points, but I think it is still a rough approximation and not exactly what I had in mind. I like the finite difference method because it is simple. However, the boundary conditions are annoying. In the book of Jim Gatheral, the Volatility Surface, he proposes a kind of analytical approach to find the risk neutral density by implicitly giving a formula for the volatility curve. However, that is quite messy as well. Since this is not the major concern of my paper, I would like to have a simple approach. Aug 8 '16 at 8:23
• Personally, I also perform a "closed-form" estimation of the pdf by relying on a parametric representation of the vol smile (usually Gatheral's SVI). The reason is that, although simple, finite difference is prone to numerical errors, with a lot of instabilities depending on your mesh size (+ adapt formulas if the mesh is not uniform) and your interpolation scheme (the manner in which you interpolate implied volatilities in your case for instance). Not to mention that careless interpolation / wrong boundary conditions can lead to arbitrage opportunities, even if there are none in the raw data. Aug 8 '16 at 8:31
• Maybe you could post your new results so someone can help you further? Aug 8 '16 at 8:33
• If it is not too much work, I will definitely take a look into the SVI parameterization by Gatheral. It is indeed true that I did not paid much attention to the nature of the interpolation etc. because I just needed a rough estimate. Thanks! Aug 8 '16 at 9:26
• Thanks for the effort @Quantuple. I guess, I'm too unexperienced in financial engineering to saw that one coming. I'll analyze the code and see what I obtain. Really helpful! Aug 9 '16 at 9:48