age) and three predictor variables (height, age and weight) and R$^2$, adjusted R$^2$ and R$^2$ change on entering each predictor variable were calculated for each model. If the R$^2$ change at each step was significant and there was a substantial improvement in the predictive ability, the model was accepted. If not, the model at the previous step was accepted. Standard errors of the estimate (SEE) were calculated and analysis of variance was carried out for each model to evaluate the significance of the regression equation. Estimates of regression coefficients were obtained and their significance was determined by student's `t' test.

The goodness of fit was examined by testing for independence of predictor variables (by examining the tolerance statistics and VIF values), normality and equal variances of the residuals (by residuals normality plot), and linear type relationship between the predictors and the outcome variable (by Q-Q plots). Unusual and influential observations were examined. These included outliers, points with high leverage and high influence. Analysis was repeated excluding these observations to determine their impact on the models and the original models were retained if the effect on the equations was small and inconsequential.

\section{RESULTS}

Acceptable data was obtained in 365 boys and 305 girls. The ages and the anthropometric data (mean$\pm$SD) in the two genders were respectively: age (years), 11.53$\pm$3.37 and 11.74$\pm$3.23 (p$>$0.05); height (m), 1.49$\pm$0.18 and 1.45$\pm$0.14 (p$<$0.01); weight (Kg), 44.56$\pm$18.42 and 40.97$\pm$13.82 (p$<$0.01); and body mass index (Kg/m$^2$) 19.04$\pm$4.43 and 18.75$\pm$3.91 (p$>$0.05). The age distribution is shown in table \ref{table_1}. The median, (range) and interquartile ranges (IQR) in boys were as follows: age (years): 11, (6 to 17), IQR: 9 to 14; height (m): 1.5, (1.09 to 1.91), IQR: 1.35 to 1.65; weight (Kg): 44, (15 to 98), IQR: 28 to 58. The median, (range) and IQR in girls were as follows: age (years): 11, (6 to 17), IQR: 9 to 15; height (m): 1.48, (1.09 to 1.7), IQR: 1.35 to 1.58; weight (Kg): 41, (15 to 78), IQR: 30 to 51.

The pulmonary functions of boys and girls were compared for the entire sample (Table \ref{table_2}). FVC, FEV$_1$ and PEFR were significantly higher among boys. However, the flow rates, F$_{50}$, F$_{75}$ and F$_{25-75}$ were not significantly different. Table 3 shows the correlations of FVC and FEV$_1$ with age, height and weight in boys and girls. Table 4 shows the correlations of PEFR, F$_{25-75}$, F$_{50}$ and F$_{75}$ with age, height and weight in boys and girls. The regression equations developed are given in table 5. The equation is expressed as follows:

Spirometric parameter $=$ Constant $+$

($\beta$ Coefficient for height $\times$ height in cm) $+$

($\beta$ Coefficient for age $\times$ age in years) $+$

($\beta$ Coefficient for weight $\times$ weight in Kg)

The SEE is provided for calculation of the lower limits of normal. As FEV$_1$/FVC was found to be independent of age and height in boys, and had only a weak correlation with age in girls, no equations were developed for this parameter.

\begin{table}[h]
\centering
\caption{Age distribution of the children}
\label{table_1}
\begin{tabular}{ccc}
Age (in years) (n) & Boys (n=365) & Girls (n=305) \\
6 (62) & 40 & 22 \\
7 (28) & 16 & 12 \\
8 (39) & 21 & 18 \\
9 (71) & 36 & 35 \\
10 (47) & 25 & 22 \\
11 (96) & 51 & 45 \\
12 (44) & 22 & 22 \\
13 (60) & 30 & 30 \\
14 (59) & 37 & 22 \\
15 (59) & 33 & 26 \\
16 (59) & 30 & 29 \\
17 (46) & 24 & 22 \\
\end{tabular}
\end{table}

\begin{table}[h]
\centering
\caption{Pulmonary function distribution in boys and girls for the whole sample}
\label{table_2}
\begin{tabular}{llllll}
Spirometry Parameter & Gender & Mean$\pm$SD & Median & Range & Interquartile Range \\
FVC & Boys & 2.88$\pm$1.09* & 2.61 & 1.09 to 6.40 & 2.03 to 3.71 \\
 & Girls & 2.42$\pm$0.72 & 2.41 & 0.83 to 4.32 & 1.84 to 2.93 \\
FEV$_1$ & Boys & 2.43$\pm$0.94* & 2.24 & 0.92 to 5.24 & 1.66 to 3.19 \\
 & Girls & 2.14$\pm$0.65 & 2.14 & 0.81 to 3.96 & 1.60 to 2.64 \\
PEFR & Boys & 5.47$\pm$2.02** & 5.04 & 1.75 to 12.00 & 3.90 to 6.86 \\
 & Girls & 5.00$\pm$1.46 & 4.97 & 2.05 to 9.92 & 3.94 to 6.05 \\
F$_{25-75}$ & Boys & 3.04$\pm$1.10ns & 2.82 & 0.97 to 6.60 & 2.21 to 3.70 \\
 & Girls & 2.97$\pm$0.91 & 2.89 & 1.24 to 5.46 & 2.29 to 3.56 \\
F$_{50}$ & Boys & 3.22$\pm$1.24ns & 2.86 & 1.04 to 7.13 & 2.32 to 3.99 \\
 & Girls & 3.17$\pm$1.04 & 3.02 & 1.00 to 5.82 & 2.33 to 3.99 \\
F$_{75}$ & Boys & 1.45$\pm$0.75ns & 1.22 & 0.34 to 4.43 & 0.92 to 1.84 \\
 & Girls & 1.52$\pm$0.66 & 1.41 & 0.33 to 3.86 & 1.00 to 1.92 \\
\end{tabular}

FVC=Forced vital capacity; FEV$_1$=Forced expiratory volume in one second; PEFR=Peak expiratory flow rate; F$_{25-75}$=Mean forced expiratory flow rate over the middle 50\% of the vital capacity; F$_{50}$ and F$_{75}$=Forced expiratory flow rates at 50\% and 75\% exhalation of vital capacity; *=p$<$0.001; **=p$<$0.01; ns=Not significant, p$>$0.05.
\end{table}