function addAssignmentsLayerToMap(visibility){
    var layerName = "All Assignments";
    var allYearFilter = new OpenLayers.Filter.Comparison({type: '~', property: 'startDate', value: '2013'});
    var allFilterStrategy =  new OpenLayers.Strategy.Filter({filter: allYearFilter});

    mapVars.assignmentLayer = new OpenLayers.JsonLayer(layerName, {
     location: '/assignments/index.json?mode=all',
     attributeMap: ['fullName', 'startDate', 'city']
    },
    {
     strategies: [
      new OpenLayers.Strategy.Cluster({distance:50, threshold:2, clustering:true}),
      allFilterStrategy
     ],
     styleMap: mapVars.clusterStyleMap
    });

    mapVars.map.addLayer(mapVars.assignmentLayer);
    mapVars.assignmentLayer.setVisibility(visibility);
 }

strategies: [ new OpenLayers.Strategy.Cluster({distance:50}) ]

strategies: [
      new OpenLayers.Strategy.Cluster({distance:50}),
      allFilterStrategy
     ]

strategies: [
      new OpenLayers.Strategy.Cluster({distance:50, threshold:2, clustering:true}),
      allFilterStrategy
     ]