我正在尝试解决Ames Housing Data Challenge，并删除了包含 NA 值大于 5% 的列，并进一步消除了具有 NA 值的其他行。
我正在尝试使用插入符号消除线性模型的特征。我已经设置了以下代码，
control <- rfeControl(functions=lmFuncs, method="cv", number=5)
results <- rfe(x[, 1:ncol(x)-1], x[, "SalePrice"], sizes=c(1:ncol(x)-1), rfeControl=control)
导致以下错误，
{ 中的错误：任务 1 失败 -“对比只能应用于具有 2 个或更多级别的因素”
事实是，x 的所有因子的水平都大于 2！
这是数据的str输出，
'data.frame': 1460 obs. of 75 variables:
$ MSSubClass : int 60 20 60 70 60 50 20 60 50 190 ...
$ MSZoning : Factor w/ 5 levels "C (all)","FV",..: 4 4 4 4 4 4 4 4 5 4 ...
$ LotFrontage : int 65 80 68 60 84 85 75 NA 51 50 ...
$ LotArea : int 8450 9600 11250 9550 14260 14115 10084 10382 6120 7420 ...
$ Street : Factor w/ 2 levels "Grvl","Pave": 2 2 2 2 2 2 2 2 2 2 ...
$ LotShape : Factor w/ 4 levels "IR1","IR2","IR3",..: 4 4 1 1 1 1 4 1 4 4 ...
$ LandContour : Factor w/ 4 levels "Bnk","HLS","Low",..: 4 4 4 4 4 4 4 4 4 4 ...
$ Utilities : Factor w/ 2 levels "AllPub","NoSeWa": 1 1 1 1 1 1 1 1 1 1 ...
$ LotConfig : Factor w/ 5 levels "Corner","CulDSac",..: 5 3 5 1 3 5 5 1 5 1 ...
$ LandSlope : Factor w/ 3 levels "Gtl","Mod","Sev": 1 1 1 1 1 1 1 1 1 1 ...
$ Neighborhood : Factor w/ 25 levels "Blmngtn","Blueste",..: 6 25 6 7 14 12 21 17 18 4 ...
$ Condition1 : Factor w/ 9 levels "Artery","Feedr",..: 3 2 3 3 3 3 3 5 1 1 ...
$ Condition2 : Factor w/ 8 levels "Artery","Feedr",..: 3 3 3 3 3 3 3 3 3 1 ...
$ BldgType : Factor w/ 5 levels "1Fam","2fmCon",..: 1 1 1 1 1 1 1 1 1 2 ...
$ HouseStyle : Factor w/ 8 levels "1.5Fin","1.5Unf",..: 6 3 6 6 6 1 3 6 1 2 ...
$ OverallQual : int 7 6 7 7 8 5 8 7 7 5 ...
$ OverallCond : int 5 8 5 5 5 5 5 6 5 6 ...
$ YearBuilt : int 2003 1976 2001 1915 2000 1993 2004 1973 1931 1939 ...
$ YearRemodAdd : int 2003 1976 2002 1970 2000 1995 2005 1973 1950 1950 ...
$ RoofStyle : Factor w/ 6 levels "Flat","Gable",..: 2 2 2 2 2 2 2 2 2 2 ...
$ RoofMatl : Factor w/ 8 levels "ClyTile","CompShg",..: 2 2 2 2 2 2 2 2 2 2 ...
$ Exterior1st : Factor w/ 15 levels "AsbShng","AsphShn",..: 13 9 13 14 13 13 13 7 4 9 ...
$ Exterior2nd : Factor w/ 16 levels "AsbShng","AsphShn",..: 14 9 14 16 14 14 14 7 16 9 ...
$ MasVnrType : Factor w/ 4 levels "BrkCmn","BrkFace",..: 2 3 2 3 2 3 4 4 3 3 ...
$ MasVnrArea : int 196 0 162 0 350 0 186 240 0 0 ...
$ ExterQual : Factor w/ 4 levels "Ex","Fa","Gd",..: 3 4 3 4 3 4 3 4 4 4 ...
$ ExterCond : Factor w/ 5 levels "Ex","Fa","Gd",..: 5 5 5 5 5 5 5 5 5 5 ...
$ Foundation : Factor w/ 6 levels "BrkTil","CBlock",..: 3 2 3 1 3 6 3 2 1 1 ...
$ BsmtQual : Factor w/ 4 levels "Ex","Fa","Gd",..: 3 3 3 4 3 3 1 3 4 4 ...
$ BsmtCond : Factor w/ 4 levels "Fa","Gd","Po",..: 4 4 4 2 4 4 4 4 4 4 ...
$ BsmtExposure : Factor w/ 4 levels "Av","Gd","Mn",..: 4 2 3 4 1 4 1 3 4 4 ...
$ BsmtFinType1 : Factor w/ 6 levels "ALQ","BLQ","GLQ",..: 3 1 3 1 3 3 3 1 6 3 ...
$ BsmtFinSF1 : int 706 978 486 216 655 732 1369 859 0 851 ...
$ BsmtFinType2 : Factor w/ 6 levels "ALQ","BLQ","GLQ",..: 6 6 6 6 6 6 6 2 6 6 ...
$ BsmtFinSF2 : int 0 0 0 0 0 0 0 32 0 0 ...
$ BsmtUnfSF : int 150 284 434 540 490 64 317 216 952 140 ...
$ TotalBsmtSF : int 856 1262 920 756 1145 796 1686 1107 952 991 ...
$ Heating : Factor w/ 6 levels "Floor","GasA",..: 2 2 2 2 2 2 2 2 2 2 ...
$ HeatingQC : Factor w/ 5 levels "Ex","Fa","Gd",..: 1 1 1 3 1 1 1 1 3 1 ...
$ CentralAir : Factor w/ 2 levels "N","Y": 2 2 2 2 2 2 2 2 2 2 ...
$ Electrical : Factor w/ 5 levels "FuseA","FuseF",..: 5 5 5 5 5 5 5 5 2 5 ...
$ X1stFlrSF : int 856 1262 920 961 1145 796 1694 1107 1022 1077 ...
$ X2ndFlrSF : int 854 0 866 756 1053 566 0 983 752 0 ...
$ LowQualFinSF : int 0 0 0 0 0 0 0 0 0 0 ...
$ GrLivArea : int 1710 1262 1786 1717 2198 1362 1694 2090 1774 1077 ...
$ BsmtFullBath : int 1 0 1 1 1 1 1 1 0 1 ...
$ BsmtHalfBath : int 0 1 0 0 0 0 0 0 0 0 ...
$ FullBath : int 2 2 2 1 2 1 2 2 2 1 ...
$ HalfBath : int 1 0 1 0 1 1 0 1 0 0 ...
$ BedroomAbvGr : int 3 3 3 3 4 1 3 3 2 2 ...
$ KitchenAbvGr : int 1 1 1 1 1 1 1 1 2 2 ...
$ KitchenQual : Factor w/ 4 levels "Ex","Fa","Gd",..: 3 4 3 3 3 4 3 4 4 4 ...
$ TotRmsAbvGrd : int 8 6 6 7 9 5 7 7 8 5 ...
$ Functional : Factor w/ 7 levels "Maj1","Maj2",..: 7 7 7 7 7 7 7 7 3 7 ...
$ Fireplaces : int 0 1 1 1 1 0 1 2 2 2 ...
$ GarageType : Factor w/ 6 levels "2Types","Attchd",..: 2 2 2 6 2 2 2 2 6 2 ...
$ GarageYrBlt : int 2003 1976 2001 1998 2000 1993 2004 1973 1931 1939 ...
$ GarageFinish : Factor w/ 3 levels "Fin","RFn","Unf": 2 2 2 3 2 3 2 2 3 2 ...
$ GarageCars : int 2 2 2 3 3 2 2 2 2 1 ...
$ GarageArea : int 548 460 608 642 836 480 636 484 468 205 ...
$ GarageQual : Factor w/ 5 levels "Ex","Fa","Gd",..: 5 5 5 5 5 5 5 5 2 3 ...
$ GarageCond : Factor w/ 5 levels "Ex","Fa","Gd",..: 5 5 5 5 5 5 5 5 5 5 ...
$ PavedDrive : Factor w/ 3 levels "N","P","Y": 3 3 3 3 3 3 3 3 3 3 ...
$ WoodDeckSF : int 0 298 0 0 192 40 255 235 90 0 ...
$ OpenPorchSF : int 61 0 42 35 84 30 57 204 0 4 ...
$ EnclosedPorch: int 0 0 0 272 0 0 0 228 205 0 ...
$ X3SsnPorch : int 0 0 0 0 0 320 0 0 0 0 ...
$ ScreenPorch : int 0 0 0 0 0 0 0 0 0 0 ...
$ PoolArea : int 0 0 0 0 0 0 0 0 0 0 ...
$ MiscVal : int 0 0 0 0 0 700 0 350 0 0 ...
$ MoSold : int 2 5 9 2 12 10 8 11 4 1 ...
$ YrSold : int 2008 2007 2008 2006 2008 2009 2007 2009 2008 2008 ...
$ SaleType : Factor w/ 9 levels "COD","Con","ConLD",..: 9 9 9 9 9 9 9 9 9 9 ...
$ SaleCondition: Factor w/ 6 levels "Abnorml","AdjLand",..: 5 5 5 1 5 5 5 5 1 5 ...
$ SalePrice : int 208500 181500 223500 140000 250000 143000 307000 200000 129900 118000 ...
请帮忙！