diff --git a/src/TR.jl b/src/TR.jl
index c79eb94..80ca581 100644
--- a/src/TR.jl
+++ b/src/TR.jl
@@ -64,7 +64,7 @@ end
 """
     function TRLooCV
 
-bpress, bgcv, press, GCV, idminPRESS, lambdaPRESS, idminGCV, lambdaGCV = TRLooCV(X, y, lambdas, regType="L2", regParam1=1, regParam2=1)
+bpress, bgcv, rmsecv, GCV, idminPRESS, lambdaPRESS, idminGCV, lambdaGCV = TRLooCV(X, y, lambdas, regType="L2", regParam1=1, regParam2=1)
 
 regType: 'bc', 'legendre', 'L2', 'std', 'GL'
 """
@@ -109,6 +109,7 @@ H           = broadcast(.+, U.^2 * broadcast(./, s, denom), 1/n);
 resid       = broadcast(.-, y, U * broadcast(./, s .* (U'*y), denom));
 rescv       = broadcast(./, resid, broadcast(.-, 1, H));
 press       = vec(sum(rescv.^2, dims=1));    
+rmsecv      = sqrt.(1/n .* press);
 GCV         = vec(broadcast(./, sum(resid.^2, dims=1), mean(broadcast(.-, 1, H), dims=1).^2));
     
 idminPRESS  = findmin(press)[2][1]; # First index selects coordinates, second selects '1st coordinate'
@@ -126,5 +127,91 @@ end
 bpress = bcoeffs[:, idminPRESS]
 bgcv   = bcoeffs[:, idminGCV]
 
-return bpress, bgcv, press, GCV, idminPRESS, lambdaPRESS, idminGCV, lambdaGCV
+return bpress, bgcv, rmsecv, GCV, idminPRESS, lambdaPRESS, idminGCV, lambdaGCV
+end
+
+"""
+    function TRSegCV(X, y, lambdas, cv, regType="L2", regParam1=0, regParam2=1e-14)
+
+Segmented cross-validation based on the Sherman-Morrison-Woodbury updating formula.
+Inputs:
+    - X : Data matrix
+    - y : Response vector
+    - lambdas : Vector of regularization parameter values
+    - cv : Vector of length n indicating segment membership for each sample
+    - regType, regParam1, regParam2 : Inputs to regularizationMatrix function
+
+Outputs: b_lambda_min, rmsecv, lambda_min, lambda_min_ind
+"""
+function TRSegCV(X, y, lambdas, cv, regType="L2", regParam1=0, regParam2=1e-14)
+
+n, p = size(X);
+mX   = mean(X, dims=1);    
+X    = X .- mX;
+my   = mean(y);    
+y    = y .- my; 
+
+if regType == "bc"
+    regMat  = [I(p); zeros(regParam1,p)]; for i = 1:regParam1 regMat = diff(regMat, dims = 1); end
+elseif regType == "legendre"
+    regMat  = [I(p); zeros(regParam1,p)]; for i = 1:regParam1 regMat = diff(regMat, dims = 1); end
+    P, _    = plegendre(regParam-1, p);   
+    regMat[end-regParam1+1:end,:] = sqrt(regParam2) * P;
+elseif regType == "L2"
+    regMat = I(p);
+elseif regType == "std"
+    regMat = Diagonal(vec(std(X, dims=1)));
+elseif regType == "GL" # Grünwald-Letnikov fractional derivative regulariztion
+    # regParam1 is alpha (order of fractional derivative)
+    C = ones(p)*1.0;
+    for k in 2:p
+        C[k] = (1-(regParam1+1)/(k-1)) * C[k-1];
+    end
+
+    regMat = zeros(p,p);
+
+    for i in 1:p
+        regMat[i:end, i] = C[1:end-i+1];
+    end
+end
+
+
+X       = X / regMat;
+U, s, V = svd(X, full=false);
+
+n_seg     = maximum(cv);
+n_lambdas = length(lambdas);
+my        = mean(y);
+y         = y .- my;
+
+denom  = broadcast(.+, broadcast(./, lambdas, s'), s')'; 
+resid  = broadcast(.-, y, U * broadcast(./, s .* (U'*y), denom));
+rescv  = zeros(n, n_lambdas);
+sdenom = sqrt.(broadcast(./, s, denom))';
+
+for seg in 1:n_seg
+        
+    Useg = U[vec(cv .== seg),:];
+    Id   = 1.0 * I(size(Useg,1)) .- 1/n;
+        
+    for k in 1:n_lambdas
+        Uk                        = Useg .* sdenom[k,:]';
+        rescv[vec(cv .== seg), k] = (Id - Uk * Uk') \ resid[vec(cv .== seg), k];
+    end
+end
+
+press  = sum(rescv.^2, dims=1)';
+rmsecv = sqrt.(1/n .* press);
+bcoeffs = V * broadcast(./, (U' * y), denom);
+bcoeffs = regMat \ bcoeffs;
+
+if my != 0
+    bcoeffs = [my .- mX*bcoeffs; bcoeffs];
+end
+
+lambda_min, lambda_min_ind = findmin(rmsecv)
+lambda_min_ind             = lambda_min_ind[1]
+b_lambda_min               = bcoeffs[:,lambda_min_ind]
+    
+return b_lambda_min, rmsecv, lambda_min, lambda_min_ind
 end
\ No newline at end of file
diff --git a/src/Ting.jl b/src/Ting.jl
index ec47504..cf8e1e5 100644
--- a/src/Ting.jl
+++ b/src/Ting.jl
@@ -18,6 +18,8 @@ export importData
 
 # From "TR.jl"
 export TRLooCV
+export TRSegCV
+export regularizationMatrix
 
 include("convenience.jl")
 include("TR.jl")