The measures to be submitted to the electors must be grouped and classified as constitutional measures, initiated statutes, or referred statutes and must be placed within such groups or classifications by the secretary of state in the order received, for the purpose of placing them on the ballot. Measures submitted by the legislative assembly must be placed first on the ballot within their classification in the order approved by the legislative assembly. Constitutional measures shall be placed first on the ballot, initiated statutes second, and referred statutes third. After all the measures have been placed within the appropriate group or classification, all measures must be numbered consecutively, without regard to the various groups or classifications.