OPINION
CONNOR, Justice.This case concerns the interpretation of certain Alaska statutes which govern the *584retirement system of school teachers. The ultimate question in this case is whether Casperson had five or more “membership years” so as to qualify her for involuntary retirement.
Appellant suffers from multiple sclerosis. On advice of her physician, she terminated employment with the Alaska Department of Education on June 30, 1978. At that time appellant had taught in Alaska as a classroom teacher in the school years of 1963-64, 1973-74, 1974-75, and as a correspondence study teacher in 1975-76, 1976-77 and 1977-78. In these last three years her contract called for four hour days, five days a week, 12 months a year. At times she worked for more than the hours called for in the contract and was paid overtime for those hours.
The Division of Retirement and Benefits [“Division”] determined that appellant’s “membership years” amounted to 4.6 instead of 6.0. The Division reasoned that appellant was only entitled to half credit for the last three years she worked. That decision was appealed to the Alaska Teachers’ Retirement Board [“Board”], and was affirmed. The superior court affirmed that decision, and this appeal was taken.
Appellant has raised six issues on appeal, but we find one issue to be disposi-tive: did the Board err when it interpreted the term “membership years” in AS 14.25.-130(a) as being the equivalent of “years of service” as defined in AS 14.25.220(16)?1
The statutes which pertain to this case should be read as a whole in order to comprehend the overall nature of the teachers’ retirement system. See AS 14.25.050(a) (am. § 1 ch. 138 SLA 1970); AS 14.25.110(a) (am. §§ 1-3 ch. 17-3 SLA 1975); AS 14.25.-115(a) (am. § 13 ch. 136 SLA 1978); AS 14.25.120(c) (am. §§ 4-6, 14 ch. 173 SLA 1975); AS 14.25.130 (am. § 7 ch. 173 SLA 1975); AS 14.25.135(a) (am. § 8 ch. 173 SLA 1975); AS 14.25.137(a) (am. § 9 ch. 173 SLA (1975); AS 14.25.140 (am. § 10 ch. 173 SLA 1975); AS 14.25.220 (am. §§ 12, 13 ch. 173 SLA 1975). In the body of this opinion it will only be necessary to quote certain key provisions of these statutes.
Appellant contends that AS 14.25.130 uses five “membership years” as the test of eligibility for involuntary retirement. This is correct, as the statute, at the relevant time, stated:
“Eligibility for disability retirement. (a) A teacher in membership service who has become permanently disabled, as defined in § 220 of this chapter, before age 55 and who has had five or more membership years may be retired by the administrator as of the first day of the month following the permanent disability.”
It appears that “membership year” is not defined elsewhere in the relevant statutes, although AS 14.25.220(5), at the relevant time, defined “membership service” as follows:
“ ‘[Membership service’ means service as a teacher in a public school within the Territory or State of Alaska, or both, under the supervision and control of the Territorial Board of Education or the Department of Education, the school board of any city or borough school district, or the Board of Regents of the University of Alaska, or any period during which the teacher is on an approved sabbatical leave granted in accordance with AS 14.20.310 or is receiving a disability retirement salary.”
*585However, the Division interpreted “membership year” as meaning “year of service” as defined by AS 14.25.220(16). The pertinent provision of this latter statute, the last clause thereof, states:
“... [I]f service is performed on a part-time basis, one-half credit shall be given for each day of service.”
As appellant points out, the Division’s interpretation means that appellant is credited with 4.6 “membership years” instead of 6. She argues that “membership year” simply means (a) membership, and (b) teaching in Alaska’s public schools for a school year. She supports this argument by asserting that if the legislature had intended “year of service” to be equated with “membership year,” it would have chosen different language from that which it used.
Under a literal reading of the statutes, the eligibility for voluntary retirement is determined by “years of membership service.” AS 14.25.110. But for involuntary retirement the term “years of membership service” is not used, and the statutes mention only “membership years.”
Next, appellant points out that benefits are calculated differently under the schemes for voluntary and involuntary retirement. For voluntary retirement the amount is computed at 2% of the average base salary, multiplied by total years of service. AS 14.25.120(c)(1). But for involuntary retirement the benefits are simply 50% of the teacher’s base salary immediately before disability. AS 14.25.140(c). And base salary is defined as any remuneration for services rendered during any school year. AS 14.25.220(2). Thus, argues appellant, the legislature had quite different things in mind when it acted in regard to involuntary and voluntary retirement.
Lastly, appellant resorts to case law of other jurisdictions. Those cases are distinguishable in various respects and do not require extended discussion.2
In response to appellant’s arguments, the state concedes that “membership years” is not elsewhere defined, but argues that a textual analysis of the statutes militates in favor of equating “membership year” with “year of service.” However, we are unpersuaded by the state’s argument. It must be assumed that the legislature consciously used different terms for a reason, and that those terms have different referents. Otherwise, the legislature simply would have used one term, e.g., “membership year” throughout these statutes.
The state next argues that the history of teachers’ retirement legislation in Alaska supports the interpretation which it seeks. From the establishment of the teachers’ retirement system in 1945 until 1970, all retirement, whether voluntary or involuntary, was based upon “years of service” and required employment on a full-time basis.3 In 1970, the legislature amended the statutes so that part-time teachers could qualify for retirement benefits. Ch. 138, § 16, SLA 1970. However, by leaving AS 14.25.-130 unchanged, the test for eligibility for disability retirement remained the same. That is, a teacher who had “five or more membership years,” could be retired for disability.
We are unpersuaded that the legislative history of these statutes supports the state’s argument. As we have said above, we must assume that the legislature was conscious of what it was doing when it amended the statute in 1970, but left AS 14.25.130 unchanged.
We hold that appellant has more than five membership years, and that she is entitled to disability retirement. Therefore, we must reverse and remand for further proceedings consistent with this opinion.
REVERSED and REMANDED.
BURKE, C.J., not participating.. The other issues, briefly stated, are:
(a) Did the Board err in concluding that appellant was a part-time, and not a full-time, teacher?
(b) Did the Board misapply AS 14.25.220(16) to appellant, who taught shorter days but more months than a normal classroom teacher?
(c) Did the Board err in concluding that appellant could not get service credit for overtime hours that she worked, beyond her contract time?
(d) Did the Board err in concluding that appellant could not get service credit for the time she was on maternity leave and sick leave?
(e) Did the superior court err in affirming the decision of the Board?
. The cases are State ex rel. Murphy v. Board of Trustees, 168 Wis. 238, 169 N.W. 562 (Wis.1918); Moore v. Board of Trustees, 170 Wash. 175, 16 P.2d 195 (Wash.1932); and Charters v. Board of Trustees, 192 Wash. 261, 73 P.2d 508 (Wash.1937).
. In 1955 a provision for disability retirement was enacted for teachers who had five or more “membership years,” but because only full-time teachers were covered the distinction between that term and “year of service” was not important. See Ch. 145, §§ 2(c), 14, SLA 1955.