Midterm(#2(Study(Guide(POS3713(Internal(Validity(• Does(our(independent(variable(really(cause(the(change(we(see(in (dep endent(variable?((Can(you(demonstrate(X(causes(Y?)((External(Validity(• Represents(the(degree(to(which(we(can(be(confident(that(the(results(of(our(analysis( apply( not( only( to( the( participants( in( the( study,( but( also( to( the(population( more( broadly( construed( (or,( Do( your( results( from( your( study(apply(to(other(areas)((Good(Research(Designs(Account(for…(The(Unit(of(Interest(• What(is(it(I(am(studying((Temporal?spatial(variation(of(interest(Cross(Sectional(• May(compare(values(of(Y(across(units(Time(Series(• May(compare(values(of(Y(across(time(Threats(to(Causality(• History((events(that(could(effect(causality)(• Maturation((is(the(unit(of(interest(changing(by(itself?)(• Testing((they(know(it’s(a(test)(• Selection((selecting(people)(• Regression(to(the(mean((the(idea(that(there(is(a(stable(level(you(should(be(at)(Good(Research(design(ensures(internal(validity(Random(assignment(ensures(internal(validity(Random(=(controls(for(thins(I(can(measure(and(things(I(can’t(measure((Experimental(Designs(Field(Experiments(• Randomly( assign( individuals( into( groups,( but( perform( the( manipulation( in(the(real(word((best(on(external(validity)((Natural(Experiments(• An(outside(event(separates(people(into(“control”(and(“treatment”(groups((not(really(an(experiment((Statistical(Control((• Find(all(the(measures(for(alternative(variables( t hat(wouldn’t(be(X(and(include(them(in(your(model((Reliability((!Test&Retest!Method!• Apply( the( same( measurement( to( observations( at( different( periods( of( time((problem(with(method(difference(in(time)(!Alternative!Form!• Using(two(different(measures(of(the(same(concept(at(two(different(times(• Same(problem(with(TestVRetest,(difference(in(time(!Split!Halves!!• Collect(two(different(measures(of(the(same(concept(at(the(same(time(• This(way(you(know(the(concept(being(measured(has(not(changed(over(time((Bias((not(random)(• Bias(systematic(over(or(underreporting(of(values((• Reliability(is(so(important(we’d(rather(have(systematically(biased( measures(them(unreliable(measures(for(hypothesis(testing(((Validity(• Does(the(measure(accurately(represent(our(concept(• Example.(What(about(asking(how(prejudiced(someone(about(race(• Your(really(measuring(the(willingness(to(reveal&prejudice(and(not(prejudice((Face(Validity(• Would(reasonable(people(agree(Content(Validity((• What( are( the( essential( elements( to( that( concept( and( do( we( have( those(elements(Construct(Validity(• Is(our(concept(associated(with(other(measures(that(it(should(be(related(to?((Levels(of(Measurement(• The&mathematical&quality&of&the&scores&of&a&variable&Nominal!• Scores(are(labels/categories’(only;(they(are(not(numbers(• They(can(not(be(ranked(or(operated(on(by(any(mathematical(function(• Categories(must(be(mutually(exclusive(and(exhaustive(• categories?no(rankings((can(only(use(the(mode)(Ordinal!• Scores(have(some(numerical(quality(and(can(be(ranked,(though(they(are(still(categories(• The(distance(between(those(ranked(is(undefined(• *Categories(with(ranking(and(gaps(between(ranking(are(meaningless((mode,(median)(*(Interval!• Even(intervals(between(ranks(• Zero(point(is(arbitrary(((year,(temp.)(• *Real( numbers,( gaps( are( meaningful,( zero( does( not( mean( anything( (uses(mean,(median,(mode)(Ratio!• Even(intervals(between(ranks(with(a(true(zero(point((• Zero(means(something’s((uses(mean,(median,(mode)((Descriptive(Statistics((populations(and(samples)(• Any(well(defined(set(of(units(of(analysis((• Samples(are(drawn(from(a(theoretically(constituted(population(• Sample(parameters(are(estimates(of(population(parameters(• The( large( the( sample,( the( smaller( the( sampling( error( (this( means( our(estimates(of(population(parameters(are(more(precise)(• When( dealing( with( a( dataset( your( first( step( should( be( to( summarize( your(data,(these(summaries(are(called(descriptive(statistics(• The(most(suitable(descriptive(is(mode(Nominal!• Nominal(variables(can(be(described(on(their(frequency(!• How(many(cases(fall(into(a(particular(category!Ordinal!!• With(ordinal(variables(we(can(describe(the(data(in(other(ways(• Mode(–most(frequent(value(of(a(variable(in(a(dataset(• Median(–when(data(arranged(from(lowest(to(highest,(middle(value(!!With! interval&ratio! level! variables,! we! can! describe! the! “moments”! of! the!variable!!• The(moments(describe(the(central(tendency(of(a(variable(and(the(distribution(of(values(around(it!Zero(Sum(Property(• Adds(up(all(the(deviations(from(the(mean(and(the(answer(is(always(zero(Least(Square(Property(• Take(all(the(values(subtract(from(the(mean(Expected(Value((• If(we(did(this(an(infinite(number(of(times(this(is(what(we(expect((guess(the(mean(always)(Distributions((Normal(“Bell”(Curve((Negative(Skew(• Few(scores(at(the(lower(end(of(the(scale((ta il (to(the(left(Positive(Skew(• Few(scores(at(the(upper(end(of(the(scale((t a
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