function AppViewModel() {
    this.statStrength = ko.observable();
    this.statEndurance = ko.observable();
    this.statAgility = ko.observable();
    this.statSpeed = ko.observable();

    this.statWillpower = ko.observable();
    this.statInsight = ko.observable();
    this.statReasoning = ko.observable();
    this.statPerception = ko.observable();

    this.statPresence = ko.observable();
    this.statComposure = ko.observable();
    this.statManipulation = ko.observable();
    this.statBeauty = ko.observable();

    this.statBonusStrength = ko.computed(function() {
        return this.statStrength();
    }, this);
}

// Activate knockout.js
ko.applyBindings(new AppViewModel());

// Source: http://stackoverflow.com/a/1830844
function isNumber(n) {
  return !isNaN(parseFloat(n)) && isFinite(n);
}

function AppViewModel() {
    this.statStrength = ko.observable();
    this.statEndurance = ko.observable();
    this.statAgility = ko.observable();
    this.statSpeed = ko.observable();

    this.statWillpower = ko.observable();
    this.statInsight = ko.observable();
    this.statReasoning = ko.observable();
    this.statPerception = ko.observable();

    this.statPresence = ko.observable();
    this.statComposure = ko.observable();
    this.statManipulation = ko.observable();
    this.statBeauty = ko.observable();

    this.statBonusStrength = ko.computed(function() {
        return this.statStrength();
    }, this);
}

// Activate knockout.js
ko.applyBindings(new AppViewModel());

Add test to check whether a variable is a number.
Add test to check whether a variable is a number.
