Attribute Effect

• Attribute Name, required, generic.attackDamage
• Modifier Amount, required, RandomValueRange
• Modifier Operation, required, 0=ADD, 1=MULT_BASE, 2=MULT_TOTAL

For deeper explanation on the operations, see Minecraft Wiki

Example:

"Effects": [
    {
      "Attribute Name": "generic.attackDamage",
      "Modifier Amount": "[2 : 3] int",
      "Modifier Operation": 0,
      "Effect Type": "Attribute",
      "Equipment Slot Type": "HAND"
    }
]