of the shape of $p_{_\delta}$ on $\Omega_m$ we get $\sigma_8\sim \Omega_m^{0.55}$. It is of interest to inspect the constraints when either of the parameters $\Omega_m$ or $\sigma_8$ is fixed at certain values. Figure (\[fig:sig\]) shows two curves of $\Delta \chi^2$ versus $\sigma_8$ corresponding to the WMAP7 $\Omega_m=0.266$ and $0.236$ giving a minimum of $\Delta \chi^2$ in the $(\Omega_m, \sigma_8)$ plane as seen in figure (\[fig:sigom\]). Figure (\[fig:om\]) plots $\Delta \chi^2$ as a function of $\Omega_m$, for $\sigma_8$ at the WMAP7 value of $0.8$ and at $0.88$ corresponding to the minimum of $\Delta \chi^2$ in figure (\[fig:sigom\]). In each of the curves in figures (\[fig:sig\]) and (\[fig:om\]), the value of $\Delta \chi^2$ at the minumum of the curve is set to zero. Hence, the $\delta \chi^2=1$ and $4$ correspond to 68% ($1\sigma$) and 95% ($2\sigma$) CLs, respectively [@press]. These curves assume a growth index $\gamma=0.55$ as is appropriate for a $\Lambda$CDM model. Hence figure (\[fig:sig\]) gives $\sigma_8=0.86\pm 0.11$ ($1\sigma$) for $\Omega_m=0.266$ and $\gamma=0.55$. However, we see from (\[eq:sigv\]) that by varying $\gamma$ alone we get the scaling $\sigma_8(\gamma=0.55)=\sigma_8(\gamma)  \Omega_m^{\gamma-0.55}$. We can use this to set a constraint on $\gamma$ if we adopt $\sigma_8=0.8$ and $\Omega_m=0.266$ [@wmap7]. Demanding that $\sigma_8(\gamma)=0.8$, the scaling 