A search for the standard model Higgs boson produced in association with a top-quark pair in 19.5 $\mathrm{fb}^{-1}$ of 8 TeV pp collision data is presented. The analysis is optimized for $\mathrm{t}\overline{\mathrm{t}}\mathrm{H}$ events which are categorized into three channels based on the different final states: lepton + jets channel ($\mathrm{t}\overline{\mathrm{t}} \to \ell\nu \mathrm{q} \overline{\mathrm{q}}^{\prime} \mathrm{b}\overline{\mathrm{b}}$, $\mathrm{H} \to \mathrm{b}\overline{\mathrm{b}}$), dilepton channel ($\mathrm{t}\overline{\mathrm{t}} \to \ell^{+} \nu \ell^{-} \overline{\nu} \mathrm{b}\overline{\mathrm{b}}$, $\mathrm{H} \to \mathrm{b}\overline{\mathrm{b}}$) and tau channel ($\mathrm{t}\overline{\mathrm{t}} \to \ell\nu \mathrm{q} \overline{\mathrm{q}}^{\prime}\mathrm{b}\overline{\mathrm{b}}$, $\mathrm{H} \to \tau^{+}\tau^{-}$). The major background to the signal is top-quark pair production with additional jets. Multivariate analysis (MVA) techniques are used to discriminate between background and signal events. A simultaneous fit for signal and background is performed using the MVA output distributions. Assuming standard model Higgs boson branching fractions, a 95$\%$ C.L. upper limit on the $\mathrm{t}\overline{\mathrm{t}}\mathrm{H}$ production cross section is presented. In addition, the results from this analysis are combined with two earlier $\mathrm{t}\overline{\mathrm{t}}\mathrm{H}$ analyses that use independent datasets.

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