Underfull \hbox (badness 10000) in tabular

Question

Hi i have an error whenever i am trying to create a tabular using multicolumn. i want to make first column empty but i am getting an error. Could anyone help?

<<Q4o3,echo=FALSE>>=
#Data for the second quarter of 2019 by each month 
O3_2019_4th = c(35.5,37.4,34.4,29.8,26.1,16.9,14.7,6.4,8,6.7,6.2,17.1,31.2,23.9,36.6,20.1,15.4,28.6,18.7,16.8,7.8,6.2,5.6,7.3,8.9,8.9,33.6,33.6,17.5,7.8)
O3_2019_5th = c(10.7,21.2,37.7,39.3,39,27.8,31.1,29,14,12.5,33.2,36.6,25.8,10.6,5.2,6.3,6.7,12.2,16,10.9,36.9,39.6,31.7,37.5,18.9,10.6,4.2,3.4,28.1,35.8,15)
O3_2019_6th = c(8.3,6.3,8.4,4.7,3.7,8.5,10.8,9.8,10.2,9.4,4.4,7.9,10.3,25.3,30.1,48.1,28.1,9.3,5.5,5.3,7.8,7.4,8,6.4,5.8,7.2,8.6,11.9,12.3,11.6)
o3a1=mean(O3_2019_4th)
o3a2=mean(O3_2019_5th)
o3a3=mean(O3_2019_6th)
o3a4=sd(O3_2019_4th)
o3a5=sd(O3_2019_5th)
o3a6=sd(O3_2019_6th)
o3a7=min(O3_2019_4th)
o3a8=min(O3_2019_5th)
o3a9=min(O3_2019_6th)
o3a10=max(O3_2019_4th)
o3a11=max(O3_2019_5th)
o3a12=max(O3_2019_6th)
o3a13=mode(O3_2019_4th)
o3a14=mode(O3_2019_5th)
o3a15=mode(O3_2019_6th)
o3a16=var(O3_2019_4th)
o3a17=var(O3_2019_5th)
o3a18=var(O3_2019_6th)
o3a19=median(O3_2019_4th)
o3a20=median(O3_2019_5th)
o3a21=median(O3_2019_6th)
#Data for the second quarter of 2020 by each month 
O3_2020_4th = c(3.1,34.3,27.7,18.9,21.2,18.3,7.7,32.4,34.7,44.6,28.3,34.2,40.1,34.5,28.7,19,18.6,12.8,11.9,6.6,6.6,25.5,20.6,12.7,11.9,35.5,29.6,40.2,38.8,27.1)
O3_2020_5th = c(13.1,9.9,10.8,7.6,8.8,7.5,8.5,8.1,7.9,9.8,8.7,18.6,33.9,32.3,17.9,7.5,9.6,10.9,15,23.6,10.9,13.5,27.4,31.8,13.7,8.2,6.5,19.1,21,4.2,7.9)
O3_2020_6th = c(5.3,9.3,9.3,10.1,9.6,8.7,7.9,8,8.6,7.8,7.3,11.6,16,10.8,7.4,7.2,8.6,8.8,9.18,8.8,8.9,9.6,9.8,8,8.4,9.6,7.8,7.5)
o3b1=mean(O3_2020_4th)
o3b2=mean(O3_2020_5th)
o3b3=mean(O3_2020_6th)
o3b4=sd(O3_2020_4th)
o3b5=sd(O3_2020_5th)
o3b6=sd(O3_2020_6th)
o3b7=min(O3_2020_4th)
o3b8=min(O3_2020_5th)
o3b9=min(O3_2020_6th)
o3b10=max(O3_2020_4th)
o3b11=max(O3_2020_5th)
o3b12=max(O3_2020_6th)
o3b13=mode(O3_2020_4th)
o3b14=mode(O3_2020_5th)
o3b15=mode(O3_2020_6th)
o3b16=var(O3_2020_4th)
o3b17=var(O3_2020_5th)
o3b18=var(O3_2020_6th)
o3b19=median(O3_2020_4th)
o3b20=median(O3_2020_5th)
o3b21=median(O3_2020_6th)
@
\textbf{Substance O3}\
\begin{table}[h]
\begin{tabular}{llllllll}
& \multicolumn{2}{1}{4th month} & \multicolumn{2}{1}{5th month} & \multicolumn{2}{1}{6th month} \
  & 2019 & 2020 & 2019 & 2020 & 2019 & 2020 \
mean               & \Sexpr{o3a1}  & \Sexpr{o3b1}  & \Sexpr{o3a2}  & \Sexpr{o3b2}  & \Sexpr{o3a3}  & \Sexpr{o3b3} \
median             & \Sexpr{o3a19} & \Sexpr{o3b19} & \Sexpr{o3a20} & \Sexpr{o3b20} & \Sexpr{o3a21} & \Sexpr{o3b21}\
standard deviation & \Sexpr{o3a4}  & \Sexpr{o3b4}  & \Sexpr{o3a5}  & \Sexpr{o3b5}  & \Sexpr{o3a6}  & \Sexpr{o3b6} \
min                & \Sexpr{o3a7}  & \Sexpr{o3b7}  & \Sexpr{o3a8}  & \Sexpr{o3b8}  & \Sexpr{o3a9}  & \Sexpr{o3b9} \
max                & \Sexpr{o3a10} & \Sexpr{o3b10} & \Sexpr{o3a11} & \Sexpr{o3b11} & \Sexpr{o3a12} & \Sexpr{o3b12}\
mode               & \Sexpr{o3a13} & \Sexpr{o3b13} & \Sexpr{o3a14} & \Sexpr{o3b14} & \Sexpr{o3a15} & \Sexpr{o3b15}\
variance           & \Sexpr{o3a16} & \Sexpr{o3b16} & \Sexpr{o3a17} & \Sexpr{o3b17} & \Sexpr{o3a18} & \Sexpr{o3b18}

\end{tabular}
\end{table}

Answer 1

\begin{table}[h]
    \begin{tabular}{llllllll}
    & \multicolumn{2}{c}{4th month} & \multicolumn{2}{c}{5th month} & \multicolumn{2}{c}{6th month} \\\cmidrule{2-3}\cmidrule(lr){4-5}\cmidrule(lr){6-7}
    & 2019 & 2020 & 2019 & 2020 & 2019 & 2020 \\\cmidrule{2-3}\cmidrule(lr){4-5}\cmidrule(lr){6-7}\rowcolor{black!20}
    mean               & {o3a1}  & {o3b1}  & {o3a2}  & {o3b2}  & {o3a3}  & {o3b3} \\
    median             & {o3a19} & {o3b19} & {o3a20} & {o3b20} & {o3a21} & {o3b21}\\\rowcolor{black!20}
    standard deviation & {o3a4}  & {o3b4}  & {o3a5}  & {o3b5}  & {o3a6}  & {o3b6} \\
    min                & {o3a7}  & {o3b7}  & {o3a8}  & {o3b8}  & {o3a9}  & {o3b9} \\\rowcolor{black!20}
    max                & {o3a10} & {o3b10} & {o3a11} & {o3b11} & {o3a12} & {o3b12}\\
    mode               & {o3a13} & {o3b13} & {o3a14} & {o3b14} & {o3a15} & {o3b15}\\\rowcolor{black!20}
    variance           & {o3a16} & {o3b16} & {o3a17} & {o3b17} & {o3a18} & {o3b18}             
\end{tabular}

\end{table}