具有4个预测变量的线性模型

Question

我正在尝试拟合具有4个预测变量的线性模型。我面临的问题是我的代码没有估计一个参数。每当我将一个变量放在我的lm公式的最后时，它就不会估计它。我的代码是：

AllData  <- read.csv("AllBandReflectance.csv",header = T)
Swir2ref <- AllData$band7 
x1 <- AllData$X1
x2 <- AllData$X2
y1 <- AllData$Y1
y2 <- AllData$Y2

linear.model <- lm( Swir2ref ~ x1 + y1 +x2 +y2 , data = AllData )
summary(linear.model)

Call:
lm(formula = Swir2ref ~ x1 + y1 + x2 + y2, data = AllData)

Residuals:
      Min        1Q    Median        3Q       Max 
-0.027277 -0.008793 -0.000689  0.010085  0.035097 

Coefficients: (1 not defined because of singularities)
            Estimate Std. Error t value Pr(>|t|)    
(Intercept) 0.595593   0.002006 296.964   <2e-16 ***
x1          0.002175   0.003462   0.628    0.532    
y1          0.001498   0.003638   0.412    0.682    
x2          0.022671   0.018786   1.207    0.232    
y2                NA         NA      NA       NA    
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 0.01437 on 67 degrees of freedom
Multiple R-squared:  0.02876,   Adjusted R-squared:  -0.01473 
F-statistic: 0.6613 on 3 and 67 DF,  p-value: 0.5787